Agios Sostis: Marathonissi, Keri Caves, and Turtle Spotting

The big draw here is the chance to see Caretta Caretta turtles in their protected home, then swim in clear water beneath the limestone cliffs of the Keri Caves. I like the glass-bottom boat, which lets you watch the sea below without giving up a shaded seat, and I like the simple three-hour plan, with time for both a swim and Marathonisi.

The main catch is that turtle sightings are never guaranteed, and several boats may gather around one animal. Also, you cannot swim inside the caves themselves. The stop is near the caves, usually around Oasi Bay, where you can enjoy the water without entering the cave chambers.

Key points at a glance

• Glass-bottom views: Look beneath the boat for a glimpse of Zakynthos Marine Park without leaving your seat.
• Two water stops: Swim near the Keri Caves and at Oasi Bay, where the water is suited to a quick swim and snorkel.
• Marathonisi visit: Spend time on the turtle island, with only a small section of beach open to visitors.
• Wildlife with limits: You may see loggerhead turtles, but sightings depend on nature, season, and boat traffic.
• Three-hour format: The short outing fits neatly into a day in Laganas or Agios Sostis.
• Good basic value: At $29 per person, the boat ride, marine park scenery, swim stops, and island visit make a strong low-cost package.

What this Agios Sostis boat trip is really like

This is a compact coastal outing from Agios Sostis harbor in Laganas, not a full-day cruise. For $29 per person, you get a three-hour ride on a glass-bottom boat, a look at the Keri Caves, a swim stop near Oasi Bay, and time on Marathonisi.

I see the value in the combination. Many boat trips focus on either wildlife or swimming. This one gives you a little of both, with a beach stop added at the end. You will not spend hours at sea, and you will not need to build your whole day around it.

You meet the boat beside the Nautilos flag in Agios Sostis harbor. The captain welcomes you at the boat, and the host service is available in Greek and English. Look for that flag rather than wandering among the boats and hoping you have found the right one.

The boat has a shaded seating area and a rear sundeck. That matters in Zakynthos, where the midday sun can turn an open deck into a hot seat. You can sit under cover for the ride, then move aft when you want more sun and wind.

The pace is relaxed, though timing can shift. One departure had a slight delay, with advance notice and an effort by the captain to accommodate everyone. Keep a little flexibility around the start rather than booking a tight appointment immediately afterward.

Watching the sea through a glass-bottom boat

The glass-bottom feature is useful, but keep your expectations sensible. You are not entering an underwater viewing room. Instead, you get occasional views into the water as the boat moves, especially when the sea is calm and clear.

I like this setup for families and for anyone who prefers not to snorkel the whole time. Children can watch for fish and movement below, while you enjoy the coast from the shaded seating area. You can also bring snorkeling gear and inspect the water during the swim stop.

The boat ride itself gives you a changing view of the National Marine Park of Zakynthos. The protected area is best known for loggerhead turtles, but the scenery is also a reason to come. You will see rocky coast, pale limestone, open water, and the wooded slopes of Marathonisi as you approach the island.

Do not expect a guided nature lecture packed with technical detail. The quality of the trip rests more on the captain’s handling, timing, and ability to find wildlife than on a formal presentation. The captains have been praised for being friendly, funny, accommodating, and able to communicate with an international crowd.

One captain received particular praise for speaking several languages and helping different passengers feel included. Another was singled out for sharing interesting stories and making the boat ride feel easygoing. Names are not provided in the booking information, so you should not expect to select a particular captain.

The Keri Caves and the Oasi Bay swim

The boat continues toward the Keri Caves, a dramatic stretch of limestone coast reached only from the water. The cliffs and openings look especially striking from a boat, where you can see how the sea has shaped the pale rock.

The important detail is this: you do not swim inside the cave. The boat usually stops at the Keri Caves area, often around Oasi Bay, where you can jump into the water and swim close to the cliffs.

This is one of the strongest parts of the outing. The water is clear, and the swim gives you a proper break from sitting on the boat. You can bring snorkeling equipment, though the trip does not include it. A towel, swimsuit, sunscreen, sunglasses, and a hat belong in your day bag.

The stop is designed for a refreshing swim and photographs rather than a long beach stay. You may have time to snorkel in shallow water, float, or climb back aboard for a dry view of the cliffs. The exact length of the stop can vary, so listen to the captain before entering the water.

Take care around rocks. One passenger cut a foot after slipping, and the captain helped with the situation. That does not mean the stop is unsuitable, but it is a reminder to move carefully near the coast and consider water shoes if you have them. Water shoes are not listed as required, so treat them as an optional safety extra rather than a tour provision.

The phrase deep blue waters fits the setting, but the practical point is simpler: you get a scenic swim in a place that is not reachable by road. If you want a long beach session, this is not the right stop. If you want a quick swim framed by rocky Zakynthos coast, it works very well.

Searching for loggerhead turtles in protected waters

Turtle spotting is the emotional centerpiece of this trip. Marathonisi and the surrounding marine park are associated with the endangered Caretta Caretta, also called the loggerhead turtle. The island is a breeding area, and the small beach where turtles lay eggs is protected.

You may see a turtle from the boat or through the glass-bottom section. Several outings have had sightings, including more than one turtle, but no responsible operator can promise one. Nature sets the schedule.

The best advice is to treat a sighting as a bonus, not as the entire reason to pay for the boat. That makes the trip worthwhile even if the turtles remain out of view. You still have the caves, the swim, the island, and the coastal scenery.

The wildlife rules matter here. Only a small portion of Marathonisi beach is open to people because much of the shore is used as a turtle breeding ground. Stay within the area you are given, and do not expect to walk the full length of the beach.

Boat traffic can complicate turtle watching. At times, several boats may gather around one turtle, creating a crowded and uncomfortable scene. I would prefer a captain who keeps a respectful distance rather than one who chases an animal for a closer photograph. Your best wildlife experience may be a shorter view that leaves the turtle undisturbed.

Season also affects what you see. One July outing included turtle sightings, but the animals were already leaving the area as eggs hatched. That is a useful reminder that the trip’s wildlife element changes through the summer. Ask your captain to explain what is happening on the day, but do not expect the same turtle activity every month.

Time on Marathonisi, the so-called Turtle Island

Marathonisi is the second major stop. As the boat approaches, you will see pine, olive, and green oak vegetation covering the island. From the water, it looks more natural and less developed than the resort areas around Laganas.

You get free time to explore the permitted area or relax near the beach. A bar sells snacks, drinks, and ice cream, which is handy if you did not bring food. These purchases are not included, and the tour does not supply lunch.

Bring your own food and drinks if you want to keep costs down. The practical packing list is short: water, a snack, sunscreen, sunglasses, a sun hat, swimwear, a towel, and snorkeling gear. Large bags and luggage are not allowed, so use a small day bag.

The beach itself is modest. One passenger considered it an average beach rather than a highlight, and I think that is a fair expectation. Marathonisi matters more for its protected status, wooded setting, and turtle connection than for a long stretch of perfect sand.

You may also find shade on the island, which is useful if you have worried about spending the entire stop in direct sun. Still, the accessible part of the beach is small. You are not arriving at a wide-open resort beach with space to wander in every direction.

The stop gives you time to reset after the boat ride. You can buy an ice cream, take photographs, swim if conditions permit, or simply sit under shade. The exact balance of time between the swim stop and Marathonisi can vary, but the overall trip lasts about three hours.

Who gets the best value from the $29 price

At $29, this is a good-value outing if you want several Zakynthos highlights in one short trip. The price includes the glass-bottom boat ticket, the Marathonisi visit, the swim stop, and the Keri Caves area. For a half-day-sized experience without hotel transportation, that is a practical package.

You need to arrange your own way to Agios Sostis harbor. Hotel pickup and drop-off are not included, so factor in the time and cost of reaching the meeting point. The harbor location keeps the price down, but it also means this is not a door-to-door excursion.

I would recommend it most strongly to:

• First-time visitors to Zakynthos who want a quick introduction to the marine park.
• Families who need a short outing with shade, a swim, and the possibility of seeing turtles.
• Budget-conscious couples and groups who want a boat trip without paying for a full-day cruise.
• Swimmers and snorkelers who value a clear-water stop near the Keri coast.
• Photographers who want cliffs, wooded Marathonisi, and open-sea views.

I would be more cautious if your main goal is a guaranteed turtle encounter, a long beach day, or swimming inside the caves. This trip cannot promise any of those things. It is a broad sampler, not a specialized wildlife expedition or a private luxury cruise.

The free cancellation policy allows you to cancel up to 24 hours in advance for a full refund, and you can reserve now and pay later. Those terms help if your schedule is still moving, but the central decision is simpler: do you want a short boat ride with several different stops, or would you rather spend your day at one beach?

Practical advice before you board

Arrive at the Agios Sostis harbor and look for the Nautilos flag directly in front of the boat. The captain welcomes you there. Since several boats can operate in the same area, identifying the flag is more useful than relying on a general harbor description.

Wear your swimsuit before boarding if you want to make the most of the water stop. Pack a towel and sunscreen, and keep sunglasses and a hat handy for the open deck. A small bottle of water and food are sensible because onboard food is not included.

Use a small bag only. Large bags and luggage are not allowed, and there is no reason to bring more than you need for three hours.

Stay alert for the captain’s instructions around the swim stop and Marathonisi beach. The coast includes rocks, and the turtle beach has restricted access. The marine park is not just a backdrop for photographs. Its rules shape where you can swim, walk, and approach wildlife.

Should you book the Marathonisi and Keri Caves trip?

I would book this tour if you want an affordable, well-packed introduction to the waters around Laganas. The glass-bottom boat, two water-focused stops, turtle possibility, and Marathonisi visit give you plenty to do for three hours, and the shaded seating makes the ride easier in strong sun.

I would skip it only if you need certainty. Turtle sightings are not guaranteed, Marathonisi beach is limited, and the Keri Caves swim happens near the caves rather than inside them.

For most people staying in Laganas or near Agios Sostis, the $29 price makes sense. Bring a small bag, accept that wildlife follows its own timetable, and choose respectful viewing over a close-up at any cost. That is the best way to enjoy this attractive slice of Zakynthos.

FAQ

How long does the Agios Sostis boat trip last?

The trip lasts three hours. It includes the boat ride, the Keri Caves area, a swim stop at Oasi Bay, and a visit to Marathonisi.

Where does the boat leave from?

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

Can I swim inside the Keri Caves?

No. Swimming inside the cave is not allowed. The boat usually stops near the Keri Caves area, often around Oasi Bay, for swimming in the surrounding waters.

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 food and drinks.

Will I definitely see a turtle?

No. The tour gives you the chance to see loggerhead turtles in their natural habitat, but sightings are not guaranteed.

What part of Marathonisi can I visit?

Only a small portion of the beach is accessible because the island is a protected turtle breeding ground. You can use the permitted area during the free time on the island.

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.