One of the places my husband and I always snorkel is the Barracuda Hotel. If you’re staying at Casa Cascada, you can walk to the Barracuda for some awesome snorkeling right off the beach. I’ve seen stingrays, all types of reef fish, a lionfish (and a stonefish) and of course, a few Barracuda. Enjoy their awesome pool with float up bar (and a great restaurant) while you’re there too! http://www.hotelbarracuda.com
Also, try the roped off area to the right of the Money Bar. You can see rays, barracudas, octopus, turtles, crabs and lobsters. Snorkel all the way to the other end of the ropes and don’t forget to circle around the rocks and coral formations as that’s where most of the critters are hanging out.
Punta Sur Park has a great long reef to explore. Costs money to get in, but well worth it!