If you're not certified, maybe try "SNUBA"--you get to go 30-35 feet down, which is plenty, attached to a regulator/hose with a big tank up at the surface. In Mexico, there are nor forms or formalities---they pretty much just push you off the side of the boat. FWIW, I'm not sure I'd be comfortable going full SCUBA with a brief resort course....
XCaret has it all right there, some pretty decent snorkeling right off the lagoon, snuba on the reef or lagoon, sea turtles all over etc., and plenty of opportunities for an iced Corona or three on the beach....and some pretty neat stuff on Mayan culture.
John