Agios Sostis: Marathonissi, Keri Caves, and Turtle Spotting

Marathonisi, the little island known as Turtle Island, is the star of this three-hour boat trip from Agios Sostis. You’ll scan the water through the glass-bottom boat, look for Caretta Caretta turtles, swim near Oasi Bay, and see the pale limestone walls around the Keri Caves. The setting is lovely, but the turtle sighting depends on nature, not the captain’s schedule.

I like the $29 price, which covers several worthwhile stops without turning the outing into a full-day commitment. I also like the choice of shaded seats and a rear sundeck, plus the friendly captains who have been praised for keeping groups comfortable and explaining what they see. The main catch is that Marathonisi’s beach access is limited, and several boats may gather around one turtle, which can feel crowded and intrusive.

Key points at a glance

• Glass-bottom views: Look beneath the boat for a glimpse of the Marine Park’s clear water and sea life.
• Oasi Bay swim stop: You can get into the water near the Keri Caves area, though swimming inside the cave itself is not allowed.
• Marathonisi visit: Spend time on the protected island, with only a small section of beach open to the public.
• Turtle spotting: Loggerhead turtles may appear, but no sighting can be promised.
• Three-hour format: A useful half-day outing from Laganas that leaves time for other Zakynthos plans.
• Bring your own supplies: Food, drinks, sunscreen, towels, and snorkeling gear are not included.

A short boat ride with a lot packed in

From Agios Sostis harbor, you’ll look for the Nautilos flag positioned in front of the boat. The captain welcomes you aboard, and you can choose between a covered seating area and a rear sundeck. That small choice matters in the Greek sun. The shaded section is better if you want to watch the scenery without baking, while the rear deck suits anyone hoping to stretch out and work on a tan.

The boat heads toward the protected waters of Zakynthos’ National Marine Park. This is not simply a transfer to a beach. The glass-bottom panels give you something to watch below the surface, while the open sides offer views of the coast and offshore rock formations.

You should expect a relaxed sightseeing trip rather than a tightly scripted expedition. Departure can occasionally run late, although advance notice may be given. The trip lasts about three hours, but the exact balance between boat time, swimming, turtle watching, and the island stop can vary.

The captains speak Greek and English. Several visitors have praised the crew for being warm, funny, flexible, and able to communicate with a mixed-language group. One captain was also praised for helping after a passenger cut a foot on a rock, a useful reminder that the boat crew is there to assist, but you still need to watch your footing around slippery shore areas.

Seeing the Keri Caves from the water

The Keri Caves are best understood from the boat. Pale rock walls meet clear blue water, and sunlight plays across the limestone near the entrances. The boat can take you into the Keri Caves area, including Oasi Bay, where you’ll have the chance to swim.

The information for this trip is clear on one point: you cannot swim inside the cave. You may get into the water near the cave area, but the boat does not offer an underground swimming adventure. That distinction is worth knowing before you book, especially if the idea of swimming through a cave is your main reason for going.

The swim stop is one of the strongest parts of the outing. You can jump into clear water, cool off, take photographs from the boat, and use snorkeling gear if you bring your own. Some people have called the stop especially pleasant for a quick snorkel in shallow water, though conditions and the exact stopping place can change.

Bring water shoes if you have them, and take care when climbing in and out. One passenger injured a foot on a rock, so do not treat the swim stop like a polished pool deck. You’ll be stepping into a natural setting where rock and water can be slippery.

Looking for loggerhead turtles in protected waters

The National Marine Park of Zakynthos protects important nesting areas for Caretta Caretta loggerhead turtles. Marathonisi is one of the best-known places connected with these turtles, and the boat trip gives you a chance to see them in the water.

That chance is real, but it is not a guarantee. Several outings have spotted one or more turtles, while the number and timing can change with the season. In July, one group was lucky to see turtles even though some had already moved on as eggs hatched. If turtle watching is your priority, you may prefer to book earlier in the summer, but the supplied information does not promise sightings at any time of year.

A glass-bottom boat gives you another way to look. You can watch the water from above and through the viewing panels, although visibility depends on conditions and the movement of the boat. Keep your camera ready, but do not expect a wildlife show on demand.

There is also an important ethical point. Several boats can gather around a single turtle, and this has been criticized as uncomfortable for the animal and unpleasant to watch. If your boat approaches a turtle, pay attention to how the captain behaves. The best experience is a respectful sighting, not a chase. A few quiet moments at a distance are more worthwhile than crowding the animal for a closer photograph.

Marathonisi, or Turtle Island

After the cave area, the boat continues toward Marathonisi. As you approach, you’ll see pine, olive, and green oak trees covering the island. The view is more natural and wooded than the name Turtle Island might suggest.

The island’s appeal is its protected setting and the chance to step off the boat for a break. You can walk around the permitted area, swim, or buy a snack, drink, or ice cream at the bar. Bring some cash or another payment method, since food and drinks are not included in the tour.

Beach access is limited because the shore is a turtle breeding ground. Only a small section is open, so do not expect to roam freely along the entire beach. The restriction is a good thing for the turtles, but it also means the island stop may feel modest if you are imagining a long beach afternoon.

You should also set your expectations about the beach itself. Some people find it pleasant and enjoyed the roughly one-hour stop, while others consider it an average beach compared with the caves and open-water views. I would treat Marathonisi as a useful pause in the boat trip, not as the sole reason to pay for the excursion.

Shade is available on the island, which is helpful in high summer. Still, bring a hat, sunscreen, and water. The bar gives you an option to buy refreshments, but you should not rely on it as your only source of supplies.

What the $29 price really covers

At $29 per person, this is a good-value outing if you want several Zakynthos highlights in a short period. The price includes the glass-bottom boat, the Marathonisi visit, the Keri Caves viewing, and the Oasi Bay swim stop. You are paying for access to places that are reached by boat, not for a full meal or a private charter.

The value is strongest for people staying in or near Laganas who want a half-day on the water. You get scenery, a swimming stop, a chance to spot turtles, and time on an island without committing an entire day. The three-hour length also works well if you have evening plans or want to combine this with other activities.

The price is less impressive if you want a quiet, private wildlife encounter. This is a shared boat trip, and turtle watching can attract several boats. It is also not designed for long snorkeling sessions, a full beach day, or extensive exploration of Marathonisi.

Food and drinks are extra, and hotel pickup and drop-off are not included. You need to make your own way to Agios Sostis harbor and find the Nautilos flag. Large bags and luggage are not allowed, so pack lightly.

Who will enjoy this trip most

I would recommend this trip to families, couples, and first-time visitors who want an easy introduction to the coast around Zakynthos. The short duration keeps the outing manageable, and the mix of swimming, wildlife watching, and scenery gives different ages something to enjoy. One child was especially delighted by the turtles, while other visitors liked the swim and photographs just as much.

It also suits people who prefer a simple boat outing instead of a day built around strenuous activity. You can sit in the shade, use the sun deck, and choose how actively you take part. You do not need to swim to enjoy the scenery, though the swim stop is a major part of the appeal.

Snorkelers should bring their own equipment. The tour information specifically recommends snorkeling gear, but it is not included. Sunglasses, a sun hat, swimwear, a towel, sunscreen, and food or drinks are also sensible additions.

I would hesitate to recommend it to anyone seeking a private or highly controlled wildlife tour. The timing of turtle sightings, the number of boats in the area, and the length of the island stop are all outside your control. If you need guaranteed wildlife viewing or a secluded beach, this is not the right choice.

Practical tips for a better morning or afternoon

Arrive with only a small day bag. Large luggage is not allowed, and you will have limited room for loose belongings on a shared boat.

Wear swimwear under your clothes and keep your towel easy to reach. The swim stop near Oasi Bay is a key part of the trip, and you may want to get into the water soon after the boat arrives.

Pack sunscreen, sunglasses, a hat, water, and a light snack. You can buy refreshments on Marathonisi, but bringing your own gives you more control over timing and cost. Keep your phone or camera protected from splashes.

Choose shade if you are sensitive to strong sun. Pick the rear sundeck if you want more sun and open air. The boat has both options, so you do not need to spend the entire ride in one place.

Use care on rocks and at the water’s edge. The natural shore can be slick, and a passenger was injured after slipping. Water shoes are a useful extra, though they are not listed as included.

Keep your turtle expectations realistic. Watch the water, listen to the captain, and enjoy the scenery even if no turtle appears. If one does show up, a calm observation is better for the animal and more satisfying than a noisy chase by several boats.

Booking flexibility and meeting details

The meeting point is in Agios Sostis harbor, in front of the boat marked by the Nautilos flag. The captain greets you there. Hotel transportation is not included, so plan your route to the harbor separately.

The activity offers free cancellation up to 24 hours before departure for a full refund. You can also reserve now and pay later, which helps if you are still arranging your Zakynthos schedule. Starting times depend on availability.

The listed host languages are Greek and English. Feedback about communication has been especially positive, including praise for captains who handled several languages and made an effort to accommodate the whole group.

Should you book the Marathonisi and Keri Caves trip?

Book it if you want a fairly priced, three-hour boat outing with caves, swimming, island time, and a chance to see turtles. The strongest parts are the clear water near Oasi Bay, the coastal views, the glass-bottom boat, and the friendly captain-led atmosphere.

Skip it if you want a private boat, a guaranteed turtle encounter, or unrestricted time on a large beach. Marathonisi is protected, so beach access is limited, and turtle watching can become crowded when several boats gather.

For most people based around Laganas or Agios Sostis, this is a sensible way to see several marine highlights without sacrificing a whole day. Just bring sun protection, pack lightly, respect the turtles, and judge the trip by the full experience rather than by wildlife sightings alone.

FAQ

Where does the boat trip meet?

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

How long does the experience last?

The boat trip lasts about three hours. Starting times depend on availability.

Is hotel pickup included?

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

Can I swim inside the Keri Caves?

No. You can swim near the Keri Caves area, including Oasi Bay, but swimming inside the cave is not allowed.

Will I definitely see a turtle?

No. The trip gives you a chance to spot Caretta Caretta loggerhead turtles, but sightings are not guaranteed.

How much of Marathonisi beach can I visit?

Only a small section of the beach is accessible because the shore is protected as a 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.