Agios Sostis: Marathonissi, Keri Caves, and Turtle Spotting

The best part of this three-hour boat trip is the chance to see Caretta Caretta loggerhead turtles in the National Marine Park of Zakynthos, then swim in clear water beside the Keri Caves. The glass-bottom boat gives you something to watch even when you stay dry.

I like the roomy choice of shaded seating or a rear sundeck, and I like the price: at $29, you get a boat ride, a swim stop, a visit to Marathonisi, and views of one of Zakynthos’s prettiest coastal areas. The main catch is that turtle sightings are never guaranteed, and several boats may gather around one animal.

Key points

• Turtle spotting is the main event: You may see loggerhead turtles in their natural habitat near Marathonisi, though sightings vary by season and luck.
• The swim stop is near the caves: You can swim in the Keri Caves area and Oasi Bay, but swimming inside the cave itself is not allowed.
• Marathonisi has limited beach access: Only a small part of the beach is open because it is a protected turtle-breeding site.
• The boat suits hot afternoons: Pick shaded seating if you want shelter, or use the rear sundeck for sun and open views.
• The harbor meeting point is simple: Look for the Nautilos flag in Agios Sostis harbor, directly in front of the boat.
• The $29 price is strong value: Food, drinks, hotel transfers, and snorkeling gear cost extra or are not included.

Setting Out From Agios Sostis Harbor

You meet the boat at Agios Sostis harbor in Laganas. Look for the Nautilos flag in front of the vessel, where the captain will welcome you aboard. Hotel pickup is not included, so you need to make your own way to the harbor.

The boat offers two basic seating choices. You can sit under cover, useful during the hot Zakynthos sun, or head to the rear sundeck for more open air and a stronger feeling of motion. I would arrive ready to claim the style of seat you prefer, especially if shade matters to you.

The trip lasts about three hours, which is long enough to reach the main sights without taking over your whole day. That makes it easy to pair with time in Laganas or another activity on Zakynthos. A slight departure delay has happened, though advance notice was given in that case and the captain worked to accommodate everyone.

The boat has a glass bottom, but keep your expectations sensible. This is not a specialist marine biology cruise with long underwater viewing sessions. The glass lets you look below the boat as you travel, while the best moments usually come from the open-water scenery, turtle sightings, and swimming stops.

Crossing the National Marine Park

As the boat leaves Agios Sostis, you travel toward the protected waters of the National Marine Park of Zakynthos. The park protects important loggerhead turtle nesting areas, and Marathonisi is one of the key places connected with the turtles.

This protected setting gives the trip more meaning than a simple coastal ride. You are not just visiting a pretty island. You are moving through an area where rules help protect nesting beaches and young turtles.

The captain may point out turtles as you go. The boat’s glass bottom can help you look down, while the crew’s experience improves your chances of noticing movement near the surface. Still, I would treat a turtle sighting as a bonus, not a promise. One July outing produced several sightings, but the turtles were already leaving the area as eggs hatched.

That seasonal detail matters. The best time for your particular sighting is not guaranteed, and nature does not follow a tour schedule. You might see one turtle clearly, several at a distance, or none at all. The caves, swimming, and island visit keep the trip worthwhile even if the turtles stay out of view.

There is also an environmental point to keep in mind. Boats can gather around a turtle, and one account described several vessels trying to follow the same animal. If you see a turtle, enjoy the moment without encouraging close pursuit or crowding. A good sighting should not come at the animal’s expense.

Swimming Near the Keri Caves and Oasi Bay

The first major stop is in the Keri Caves area, usually around Oasi Bay and close to the cave zone. These coastal formations are reached by boat, and their pale limestone walls create strong contrasts with the blue water.

You get a chance to swim in the open water near the caves. The water is clear enough for swimming and snorkeling, and several people appreciated the quiet time to jump in, take photographs, and cool off. Bring your own snorkeling gear if you want to look beneath the surface, since it is not included.

The wording here matters: you do not swim inside the cave. The stop is in the surrounding cave area and near Oasi Bay. You may get a look into the caves from the boat and see the way light falls across the limestone, but the swimming takes place outside the cave itself.

This is probably the most active part of the trip. You can jump into the sea, swim, snorkel, or stay aboard and take pictures. If you are not a confident swimmer, use the time carefully and follow the captain’s instructions. The tour information does not promise flotation equipment, so bring only what you are comfortable using in open water.

A previous swimmer cut a foot on a rock, and the boat driver helped with the situation. That detail is a useful reminder to watch your footing near rocky water and not rush into the sea. Water shoes may be a sensible addition, though they are not listed as required equipment.

The swim stop also depends on conditions and the day’s timing. You may find the stop feels generous, while another departure may spend more time moving between sights. Either way, carry a towel, swimsuit, sunscreen, sunglasses, and a hat. A dry layer can also make the return ride more comfortable if wind picks up.

Seeing Marathonisi, Turtle Island

From the cave area, the boat continues to Marathonisi, often called Turtle Island. As you approach, you can see pine, olive, and green oak trees covering the island. The view is a welcome change from the open sea and gives you a short taste of Zakynthos’s protected coastal character.

The island visit includes time to go ashore and swim. You can walk around the accessible area, sit in the shade, or stop at the bar for a snack, ice cream, or drink. Food and beverages are not included, so bring your own supplies or plan to buy something there.

The beach is not fully open for exploring. Only a small section is accessible because the beach is a nesting site for loggerhead turtles. That restriction can feel limiting if you expect a long beach walk or a full afternoon on the island, but it is also the reason the place remains important to the park.

You should not compare Marathonisi with a large resort beach. One four-star assessment called it an ordinary beach and suggested the tour might improve with another stop. That is fair. The island’s appeal comes less from beach facilities and more from its protected setting, turtle connection, wooded views, and the chance to swim from a memorable location.

Shade is available on the island, which is useful in summer. One person specifically worried about finding shelter from the sun and was pleased with the amount of shade. Even so, bring a hat and sunscreen because you may spend part of your time in exposed areas.

The island stop is also a practical pause in the trip. After the boat ride and swim, you can slow down, get an ice cream, and enjoy the view without having to follow a demanding schedule. The break lasts around an hour in at least one departure, but exact timing can vary.

Turtle Spotting Without False Promises

Loggerhead turtles are the reason many people book this excursion. Marathonisi is a breeding ground, and seeing one in its natural setting can be a powerful moment, especially for children. Several families found the turtle sighting to be the highlight, and one child was especially excited by it.

The glass-bottom boat adds a second way to search. You can watch the water below while the captain looks ahead and around the boat. The crew may also share information and point out wildlife, scenery, and good photo angles.

Still, this is wildlife watching, not a guaranteed animal encounter. The season affects where the turtles are, and conditions on the day matter. In July, turtles may already be leaving the area as eggs hatch, so you should book for the full experience rather than for a guaranteed photograph.

Be ready with a camera, but keep your phone secure on the boat. The National Marine Park scenery gives you plenty to photograph even without a turtle. The caves, wooded island, bright water, and boat-level views all make good subjects.

The captain is an important part of the experience. Past departures have been praised for friendly, funny, and capable captains, including a captain who spoke several languages and made an effort to accommodate different passengers. No specific captain name is provided, and the crew may vary, but the captain’s local handling of the boat and willingness to help clearly affect the day.

What the $29 Price Really Covers

At $29 per person, this is a reasonably priced way to reach places you cannot visit easily from shore. The fee covers the glass-bottom boat ticket, the Marathonisi visit and swim stop, the Keri Caves area, and the Oasi Bay swim stop.

You are paying for access, not a luxury cruise. The boat provides shaded seating and a sundeck, but the experience is built around simple transport, scenery, swimming, and wildlife watching. That is exactly why the price works well for families and couples who want a short sea outing without paying for a full-day charter.

The costs you need to plan for are straightforward. Hotel transfers are not included, and neither are food or drinks. You can buy refreshments at the bar on Marathonisi, but bringing your own food and water is allowed and may help you spend less.

Snorkeling gear is also not provided. Pack your own mask and snorkel if you want to see below the surface during the swim stop. Large bags and luggage are not allowed, so keep your belongings compact and practical.

The tour has free cancellation up to 24 hours before departure, with a full refund, and you can reserve now while paying later. Starting times vary, so check availability when booking. The activity is offered in Greek and English.

Who Will Enjoy This Boat Trip Most

I would choose this excursion if you want a compact introduction to the marine park and do not want to spend a full day at sea. It gives you turtles, caves, a swim, an island visit, and a little free time in one three-hour outing.

It suits families particularly well because the route offers variety. Children can look through the glass bottom, search for turtles, swim, and get an ice cream on Marathonisi. The boat’s shaded seating also helps if younger passengers need a break from the sun.

It is a good fit for swimmers who want a simple sea stop rather than a formal lesson or guided snorkeling session. You can bring your own gear and explore at your own pace, but the activity does not promise specialist instruction.

You may want something else if your main goal is a long, quiet beach day. Marathonisi has only a small accessible portion of beach, and the beach itself may feel average compared with other coastal stops. You should also look elsewhere if you want to swim inside the Keri Caves, since that is not allowed.

People with limited time in Zakynthos can get good value from the compact schedule. You see several signature sights without giving up an entire day. Just remember that you must arrange your own transport to Agios Sostis harbor.

Practical Packing for the Three Hours

Pack light. Large bags and luggage are not allowed, and you do not need much for a short boat trip.

Bring:

• Swimsuit and towel
• Sunscreen, sunglasses, and a sun hat
• Food and drinks if you do not want to buy them on Marathonisi
• Snorkeling gear
• A small, secure bag for essentials

Keep phones, cameras, and other electronics protected from spray. The boat has shaded seating, but you should still prepare for strong sun when you are on deck or at the island.

I would also bring footwear that feels secure around rocky water. The tour does not require water shoes, but careful footing is sensible at a natural swimming stop.

Should You Book the Agios Sostis Boat Trip?

Book it if you want a good-value half-day outing with several distinct parts: a glass-bottom cruise, Keri Caves scenery, a swim near Oasi Bay, turtle spotting, and time on Marathonisi. The friendly, helpful captain can add a lot, and the three-hour format leaves room for other plans.

Skip it if you need a guaranteed wildlife sighting, a long sandy beach visit, or permission to swim inside the caves. The strongest reason to go is the combination of marine park scenery and the chance to see a loggerhead turtle, not Marathonisi’s beach alone.

For most people staying near Laganas, this is an easy and worthwhile way to get onto the water. Keep your expectations realistic, respect the turtles, pack light, and treat the wildlife encounter as luck rather than a promise.

FAQ

Where does the boat tour meet?

The meeting point is Agios Sostis harbor in Laganas. Look for the Nautilos flag directly in front of the boat, where the captain will welcome you.

How long does the tour last?

The tour lasts three hours. Starting times vary, so check availability for the date you want.

Is hotel pickup included?

No. Hotel pickup and drop-off are not included, so you need to reach Agios Sostis harbor yourself.

Is the boat glass-bottomed?

Yes. The boat has a glass bottom, allowing you to look below the water as you cruise through the National Marine Park of Zakynthos.

Can I swim inside the Keri Caves?

No. Swimming inside the cave is not allowed. You can swim in the Keri Caves area, usually around Oasi Bay and close to the cave zone.

Is a swim stop included?

Yes. The tour includes a swim stop near Oasi Bay in the Keri Caves area, as well as a visit and swim stop at Marathonisi.

Will I definitely see a turtle?

No. The tour gives you a chance to spot loggerhead turtles in their natural habitat, but sightings depend on conditions, season, and luck.

How much of Marathonisi beach can I visit?

Only a small portion of the beach is accessible because it is protected as a loggerhead turtle breeding ground.

Are food and drinks included?

No. You can buy snacks, drinks, and ice cream at the bar on Marathonisi, or bring your own food and drinks.

What should I bring?

Bring sunglasses, a sun hat, swimwear, a towel, sunscreen, food and drinks, and snorkeling gear. Large bags and luggage are not allowed.