There are several dive sites in the Playa vicinity down to around 100 feet/30m. Many, however, are around 20m. You will see most of the common Caribbean marine life. Turtles, lobsters, shrimp, Grey Angelfish, French Angelfish, Rock Beauties, Queen Anglefish, and even Blue Anglefish. Lots of parrotfish of various species, lots of French Grunts, snappers (particularly Schoolmasters), puffers and burrfish, barracuda, trumpetfish, scorpionfish, green morays, spotted morays, etc., etc. There is actually quite a lot of marine life along the coast.
Cozumel is very different. Many of the reefs are quite deep and drop off into walls that disappear into the blue thousands of feet down. A typical morning two-tank dive will start with a wall dive to 25-30m, followed by a long surface interval and a second dive that may be as deep as 20m, or as shallow as 10-15m. The marine life is pretty much the same, although I have never seen a Blue Angelfish there.