Drive north out of San Miguel and the hotels thin, then stop. The paved road runs on past the last of the marinas into scrub and mangrove, and eventually reaches a small pier on Bahía Ciega with a few boats tied to it. Five to ten minutes across the lagoon from that pier is Isla de la Pasión: a low sandy island about a kilometre long, with palms, thatched shade, a bar, a kitchen, and nothing else.
The water is the reason people come, and it is a different substance from the water on the rest of Cozumel. This is lagoon, not open Caribbean. It sits over pale sand, it is shallow enough to wade a long way out with a drink still above the waterline, and by the middle of the afternoon it is close to bath temperature. It also stays flat when the rest of the island is being blown around, because the lagoon is protected from every direction that matters.
That same shelter is the trade-off. There is no reef here, no coral, and very little to look at through a mask beyond sand and the occasional ray. Cozumel is one of the best places on earth to put your face in the water, and this is not where you do it. Come here for the water you sit in rather than the water you look through.
The other thing to be clear about is what Isla de la Pasión actually is now. It is a concession, run as a private beach club. You do not turn up, park and walk on. Access is sold as a package — the crossing, a buffet lunch, an open bar, a lounger and use of the kayaks and paddleboards — either as a beach day booked directly with the boat operator at the north pier or as an excursion from the cruise piers on a faster boat. Terms and operators here have changed more than once, so what is included is worth confirming at the point of booking rather than assuming.
What you get for it is scale and quiet. The island takes a limited number of people, the beach is long enough to walk away from all of them, and there is nothing on the horizon in any direction except the low green line of Cozumel's north shore and the odd frigatebird.