The Mediterranean off the Cote d'Azur is warm, clear and, in the right spot, surprisingly full of life. The French have a clever way of showing it off: the sentier sous-marin, a marked underwater trail you swim with a mask, snorkel and fins, following a line of numbered buoys with information panels set into the seabed below. For a cruising family it is one of the best afternoons the coast offers, and most of the good ones sit within an easy dinghy ride of an anchorage.
I have swum a handful of these over a couple of seasons. What follows is where they are, what they are like, and how to fit them around a boat.
What a sentier sous-marin actually is
The format is consistent up and down the coast. A buoyed route in shallow water, usually two to six metres deep, runs over a stretch of seabed chosen for its variety: posidonia meadow, rocky reef, the occasional sandy patch. Each buoy carries a number and often a handhold so you can stop and breathe without standing on anything living. Submerged panels name what you are looking at, the fish, the seagrass, the urchins. Some trails are free and self-guided; others run guided sessions in summer with kit supplied.
The depth is the appeal. You do not need to dive. Anyone who can swim and put their face in the water can do the whole route, which makes these perfect for kids and nervous first-timers. The trails sit inside protected zones, so the fish are used to people and far less shy than they are where spearos work.
Cap d'Antibes
The trail off the Cap d'Antibes, near the Pointe de l'Ilette and the old Graillon battery, is the one I send people to first. You can reach the seabed straight from the shore, the water is shallow and clear, and the site sits inside a marine protected area, so there is real life on the reef. It is busy in August, which is the trade-off, but the swimming is genuinely good.
From a boat, anchor off in the navigable water outside any bathing zone, watching the seagrass: this is posidonia country and the posidonia anchoring ban in France is enforced hard along this coast. Take the dinghy in to the shallows and swim the trail from there. The Lerins islands sit just along the coast, and a snorkelling stop here pairs naturally with a night at anchor off them, which I cover in the Lerins islands anchorage at Cannes piece.
The guided trail at the Rayol
Further west, towards the Var coast, the Domaine du Rayol runs a guided underwater trail in the Figuier bay. This one is escorted: you go in with a guide from the Domaine, mask, snorkel and fins supplied, and swim a route through a protected cove. It is less of a free-for-all than Antibes and better for it. Booking is needed in season.
This stretch of coast, the corniche des Maures running towards the Iles d'Hyeres, is where the cruising and the snorkelling come together best. The water hits 24 to 26 degrees by mid-August, warm enough to stay in for an hour without a wetsuit, and the anchorages are quieter than the headline names around Cannes and Saint-Tropez.
Port-Cros: the original
The grandfather of them all is the trail off the Plage de la Palud on Port-Cros, the island at the heart of France's oldest marine national park. The park created the sentier sous-marin concept here decades ago. The fish life is the best of any trail I have done on this coast precisely because the whole island has been a strict reserve for so long. Grouper, big wrasse, clouds of bream that ignore you completely.
Cruising in means understanding the Port-Cros national park mooring rules, which are tight: anchoring is restricted across much of the park to protect the seabed, and you pick up a buoy or use the designated zones. Once you are settled, the Palud trail is a short walk and swim away. The same protective regime that makes the mooring fiddly is exactly why the snorkelling is so rich.
Villefranche, Antibes and the eastern trails
East of Antibes the coast steepens and the water deepens fast, which changes the character of the snorkelling. The bay of Villefranche-sur-Mer drops away quickly from the shore into one of the deepest natural harbours on the coast, so the shallow-trail format gives way to richer rocky edges where the reef meets blue water. There is a marine education centre at Antibes built around the posidonia and the coastal ecosystem, and several local outfits run guided snorkel hikes in summer over shallows chosen for their wildlife, fins and snorkel supplied.
For a cruiser working the stretch between Cap Ferrat and Cannes, the practical move is to treat the trails as set-piece stops on a day's coastal hop rather than destinations in themselves. Anchor for lunch in a sheltered cove, swim a trail in the afternoon when the day boats thin out, move on for the evening. The corniche here is steep-to and pretty, and a snorkel stop slots neatly into the kind of coast-hugging passage you would plan along the Cap Ferrat and Villefranche bays.
What you see varies more than people expect over a short distance. The protected trails hold grouper, big wrasse and dense shoals of bream; swim somewhere unprotected a mile away and the same rock is comparatively empty. The lesson lands quickly: the fish are where the spearguns are not.
How to work them around a boat
A few practical notes from doing this off our own deck:
- Time it for calm and clarity. Morning, before the sea breeze stirs the surface, gives you the best visibility. A trail that is gin-clear at nine can be milky by three.
- Mind the bathing zones. Most trails sit inside or beside supervised beaches, which means you are in the 300-metre coastal band where boats go at 5 knots and bathing areas are buoyed off. The full rules on getting a dinghy ashore through that are in the swimming and snorkelling beach landings guide. Anchor outside, take the tender in slowly, never motor among swimmers.
- Bring your own kit if you can. Self-guided trails supply nothing. A decent mask that seals, fins, and a rash vest against the sun on your back, which fries on a long float face-down even at 25 degrees.
- Tow a float on a busy trail. You are low in the water and a passing tender will not see you. A small surface marker keeps you visible.
- Leave the seabed alone. These are protected zones. No standing on the posidonia, no touching, no taking. The trails exist because the French stopped people doing exactly that.
What you will actually see down there
For a first-timer, knowing what to look for turns a swim into a treasure hunt. The Mediterranean is not the tropics, but a good trail is far from empty once your eye tunes in.
The seagrass meadows, the posidonia, are the foundation of it all. They look like underwater fields, and they shelter the young of half the fish you will eat ashore. Over and around them you find clouds of saddled bream and salema, the occasional curious wrasse in startling colours, sand smelt shimmering near the surface, and gobies and blennies tucked into the rock. On the older protected trails like Port-Cros, the prize sightings are the big dusky grouper, slow and unbothered, and shoals of larger bream that have never been hunted and simply ignore you.
Look at the rock itself, not just the fish. Sea urchins wedge into crevices, sometimes carrying bits of shell as camouflage. Bright orange and yellow sponges, the odd anemone, and if you are lucky a moray eel's head poking from a hole. The trails put information panels on the seabed for exactly this reason: they teach you to read what you are floating over.
Bring an underwater camera or a phone in a housing if you have one. Half the pleasure for the kids is showing off what they spotted afterwards, and the shallow clear water on these trails is forgiving of an amateur snap.
What strikes me every time is how much richer these protected stretches are than the open coast. Swim a sentier sous-marin and then snorkel somewhere unprotected nearby, and the difference in fish life is stark. The trails are a free lesson in why the marine reserves matter, dressed up as a brilliant afternoon for the kids. Pick your weather, respect the seabed, get the dinghy in slowly, and the Cote d'Azur shows you a side of itself that most people who only ever see it from a sun lounger never reach.
