Agios Sostis: Marathonissi, Keri Caves, and Turtle Spotting

A glass-bottom boat, bright limestone caves, and a chance to see loggerhead turtles make this one of the easiest ways to sample Zakynthos’s protected south coast. The three-hour trip from Agios Sostis harbor packs in plenty: a swim near Keri Caves, a stop at Oasi Bay, and free time on Marathonisi.

I like the low price of $29, especially since the boat ride, turtle island visit, and swimming stop are included. I also like the choice of shaded seating or a rear sun deck, which matters on a hot Greek afternoon. The catch is that turtle sightings are never guaranteed, and several boats may gather around one animal, raising real questions about responsible wildlife watching.

Key points to know

• Glass-bottom views: Look into the water as you cruise through Zakynthos’s National Marine Park.
• Two water-focused stops: Swim at Oasi Bay and near the Keri Caves, but not inside the cave itself.
• Marathonisi access is limited: Only a small part of the beach is open because it protects turtle nests.
• A practical three-hour outing: The schedule suits a half-day plan from Laganas or Agios Sostis.
• Friendly multilingual captain: The crew speaks Greek and English, with past trips praised for clear communication and a good sense of humor.
• Bring your own basics: Pack swimwear, towel, sunscreen, snacks, drinks, and snorkeling gear.

Setting out from Agios Sostis harbor

Your meeting point is easy to identify: look for the Nautilos flag directly in front of the boat at Agios Sostis harbor. The captain welcomes you there, so you do not need to search through a large marina or find a separate office.

Hotel pickup and drop-off are not included. If you are staying in Laganas, plan your route to the harbor before departure and allow time to find the correct boat. The activity is listed as three hours, but the exact starting times depend on availability.

The boat offers two useful seating choices. You can stay under cover in the shaded area, or move to the rear sundeck for more sun and open views. I would choose shade for the main cruise and use the deck when the boat is moving or when you want clear photographs.

The vessel is described as roomy, and that is helpful during a trip that includes several stops. Still, do not bring luggage or large bags. A small day bag with your swim kit is the sensible choice.

Seeing Zakynthos through the glass bottom

The glass-bottom feature gives the ride more purpose than a simple transfer between beaches. As the boat moves through the National Marine Park of Zakynthos, you can look below the surface for a glimpse of the marine world beneath you.

Do not expect an aquarium-style display. The glass panel is best viewed as an extra way to watch the water, not as a guarantee of constant sea life. Conditions, light, and the boat’s movement all affect what you can see. The real reward is the combination of open-water scenery, clear shallows, and the possibility of spotting a turtle.

Keep your sunglasses and sun hat handy. The Greek sun can be strong even when a breeze makes the boat feel cool. Sunscreen, a towel, and swimwear are essential, while snorkeling gear can make the swimming stops more rewarding.

Swimming near the Keri Caves and Oasi Bay

The boat usually stops around the Keri Caves area and Oasi Bay, where you can get into the water. This is the most active part of the trip, so bring swimwear and expect wet steps, splashes, and a short return to the boat afterward.

The water near the caves is the attraction. Limestone walls catch the sunlight, and the blue color changes as you move from open water toward the rock formations. You can swim and take photographs from the boat, although the exact stop can depend on conditions and boat traffic.

Pay attention to one important detail: swimming inside the cave is not allowed. The trip takes you to the cave area, where you can admire the formations and swim nearby. This is still a fine experience, but you should not book it expecting to paddle into a cave tunnel.

Oasi Bay provides another chance to get into the sea. The stop has been appreciated for clear water and easy snorkeling, especially in the shallow areas. Bring your own snorkeling gear because it is listed among the items to pack, not as equipment supplied with the tour.

Use care on rocks and around the boat. One person badly cut a foot after slipping on a rock, and the captain helped with the situation. That does not make the stop unsafe, but it is a good reason to move slowly, watch your footing, and avoid scrambling across wet stone.

Searching for Caretta Caretta turtles

The turtle search is the most memorable part of the trip for many people, but it is also the part that requires the most patience and respect. The waters around Marathonisi are a breeding area for endangered Caretta Caretta loggerhead turtles.

You may spot a turtle from the boat, and the glass bottom can help you look down without entering the water. Sightings are not certain. The season and the turtles’ behavior matter. One July trip was lucky enough to see turtles, but the animals were already leaving the area as eggs began to hatch.

When a turtle appears, several boats may arrive at once. That can give you a good view, but it can also create a crowded scene in which boats follow one animal too closely. I would treat the sighting as a privilege, not a chase. Keep your distance, follow the captain’s directions, and accept that a brief glimpse is better than causing stress to a protected animal.

This is also why the tour works best when you judge it by the whole outing, not by the turtle alone. The caves, swimming, boat ride, and Marathonisi stop still give you a worthwhile morning or afternoon if the animals stay out of sight.

Landing on Marathonisi, Turtle Island

Marathonisi is often called Turtle Island because loggerhead turtles lay eggs on its beach. As you approach, you will see pine, olive, and green oak vegetation, which gives the island a more natural feel than the developed resort coast around Laganas.

The beach stop is not a typical open beach day. Only a small portion of the beach is accessible, since most of the shore is protected turtle-breeding ground. You can spend free time in the permitted area, swim, take photographs, or visit the bar for a snack, drink, or ice cream.

Food and drinks are not included. You can buy refreshments on the island, but packing your own food and water is also sensible. The activity specifically advises bringing food and drinks, which lets you avoid spending extra money during the stop.

The island’s appeal is its setting and protected status more than a long list of facilities. One person found the beach fairly ordinary and felt the tour might be stronger with a different stop. That is a fair criticism if you want a dramatic beach with lots of room to walk. Marathonisi is better suited to a short swim, a cold drink, and a look at an important nesting site.

Shade is available in parts of the island, which is useful if you are traveling with children or do not want to sit in direct sun. Still, bring a hat and sunscreen. The accessible beach area is limited, and you should stay within the permitted zone rather than wandering toward closed nesting areas.

Timing, value, and the shape of the day

At $29 per person, this is a good-value boat outing if you want several experiences in one booking. You get the glass-bottom boat ride, a visit to Marathonisi, a swimming stop, views of Keri Caves, and time around Oasi Bay. Comparable private boat trips would normally offer more control, but they also cost much more.

The three-hour length is another advantage. You can fit this trip into a day that includes other plans in Zakynthos, and you are not committing to a full-day cruise. The trade-off is a fairly brisk schedule. You will have time to swim and enjoy the island, but not enough time for a long, unhurried beach stay.

One detail is worth clarifying before you board. The Marathonisi break can last about an hour, but that timing is not always made clear in advance. If you prefer a continuous sightseeing cruise, the island stop may feel long. If you want to dry off, eat something, and enjoy a calm break between boat sections, the hour is useful.

A slight departure delay has occurred on some trips, with advance notice given in that case. Build a little flexibility into your plans rather than scheduling an important connection immediately afterward.

The captain and the tone of the trip

The captain is central to the experience. The strongest feedback focuses on a friendly, funny captain who shares stories, handles practical problems, and accommodates people speaking different languages. Greek and English are offered, and the captain can help keep the group informed during the stops.

This is not a formal historical tour with a long lecture. Expect simple guidance, local observations, and help getting the most from the water stops. That relaxed approach suits a short recreational cruise.

The crew has also been praised for helping when plans changed or small problems arose. That matters on a boat trip, where clear instructions and a calm response are more useful than polished speeches.

What to pack for this boat trip

Pack light, since large bags and luggage are not allowed. A small beach bag should be enough.

• Swimwear and a towel
• Sunglasses and a sun hat
• High-protection sunscreen
• Food and drinks, unless you plan to buy them on Marathonisi
• Snorkeling gear
• A phone or camera with a secure grip or waterproof case

Wear something that can handle sea spray. Keep valuables protected, and do not leave loose items on the sun deck. If you plan to climb onto rocks at a swimming stop, use extra care because wet stone can be slippery.

Who will enjoy this Zakynthos excursion?

I would recommend this trip to anyone staying around Laganas or Agios Sostis who wants a short, affordable boat outing without committing to a full day. Families may appreciate the mix of turtles, swimming, and ice cream, although young children should be watched closely around the boat and slippery rocks.

It also suits first-time visitors who want an introduction to the National Marine Park. You see several of the south coast’s signature sights in a single outing, and the schedule leaves room for other plans.

You may want a different option if your main goal is a quiet, uncrowded beach. Marathonisi’s open section is small, and turtle sightings can draw multiple boats together. You should also look elsewhere if you expect to swim deep inside a cave, since swimming is limited to the surrounding area.

The experience is less ideal for anyone who dislikes fixed group schedules. You cannot choose how long to spend at each stop, and the turtle search depends on nature, weather, and boat traffic.

Should you book the Agios Sostis boat trip?

Book it if you want good value, clear water, a chance to see turtles, and a taste of the Keri Caves area in three hours. The most reliable highlights are the boat scenery, the swim near Oasi Bay, the limestone formations, and the chance to spend time on protected Marathonisi.

Go in with realistic expectations. You are not guaranteed a turtle, the beach area is restricted, and the boat may share the water with other turtle-watching vessels. For $29, though, the combination is hard to fault. Bring your own snorkeling gear and refreshments, stay respectful around wildlife, and this should make a pleasant half-day addition to your Zakynthos plans.

FAQ

Where does the boat trip begin?

The meeting point is at 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 experience last?

The boat trip lasts three hours. Starting times vary, so you need to check availability when booking.

Is hotel pickup included?

No. Hotel pickup and drop-off are not included, so you must make your own way to Agios Sostis harbor.

Can I swim inside the Keri Caves?

No. Swimming inside the cave is not allowed. You can swim in the surrounding Keri Caves area and at Oasi Bay.

Is a turtle sighting guaranteed?

No. The trip gives you a chance to look for loggerhead turtles in their natural habitat, but sightings depend on conditions and turtle behavior.

How much of Marathonisi beach can I visit?

Only a small portion of the beach is accessible because the island is a protected turtle-breeding area.

Are food and drinks included?

No. Food and drinks are not included. You can buy snacks, drinks, and ice cream at the bar on Marathonisi, or bring your own.

What languages does the host speak?

The host or captain speaks Greek and English.