Playa Corona is what Cozumel's west coast looked like before the day-pass economy arrived. It sits on the rocky shoreline around kilometre 9 to 10 of the Carretera Costera Sur, between Chankanaab and Sky Reef, and it consists of a family-run palapa restaurant, a bar, a scatter of loungers and tables under thatch, a set of steps and a ladder down to the water, and the reef.
That reef is the point and everything else is scaffolding around it. The fringing coral here starts a short swim from the ladder and runs parallel to the coast, shallow enough that you see it all from the surface, with the usual western Caribbean cast: stoplight and queen parrotfish, sergeant majors and blue tangs in shoals, angelfish along the ledges, damselfish guarding their patches, snapper hanging off the overhangs and southern stingrays on the sand channels between coral heads. In the cool months, eagle rays pass along this coast. It is the same reef system the Money Bar and Sky Reef sit on, worked by the same north-running current.
What Playa Corona does differently is stay small and stay cheap. There has never been an all-inclusive wristband here. Historically you turn up, order something from the restaurant, use a lounger and the ladder, and that is the transaction. A couple can spend a long morning snorkelling and eat a proper lunch for less than one all-inclusive pass at kilometre 15. Confirm the current arrangement when you arrive, because small operators on this island do adjust, but it has worked this way for a long time.
The honest limitations are real. It is small enough that one excursion group fills it. Shade is palapa thatch and whatever trees are standing, not a landscaped resort. Facilities are basic. There is no pool, no waterslide, no beach volleyball and no beach to speak of — you get in over rock. None of that is a complaint. It is a description of what you are choosing, and a lot of people who have done Cozumel three or four times end up choosing exactly this.