Cwaver, I don't know of dive shops in Playa specifically, but I'd say just pick some cenotes and go! I've gone cenote-hopping with my wife and son (now 8) on several trips to the Yucatan during the past few years, and we've always had a great time snorkeling. The Gran Cenote off the road from Tulum to Coba is probably our favorite, although we also liked the Crystal Cenote south of Tulum, Dos Ojos near Akumal and numerous others. (There's a really spectacular one on the road to Chichen Itza, but that's quite a bit inland.) We brought our own masks and snorkels, but I'm sure there are lots of places you can rent them. We've gotten around to the cenotes both by taxi and rental car. If you really want to do it as a shop tour, there are probably some that can pick you up in Playa. Have fun!