Akumal Bay has Sea turtles, there is a tour but you can go for free if you take your own snorkeling gear and just make your own way, there is a local bus you can take to Akumal from the station on 5th Ave in down town Playa Del Carmen. There is some rather commercial snorkeling in Xel-ha park which has swimming with dolphins and floating down the river on tubes. Also a decent Dive shop in Playa, this should help, have fun!
http://www.diveshopmexico.com/