We visited Costa Maya in July of 2001. There was a public beach area right off of the dock. At the end of the dock there was a market area and pool area as well. I did talk to some people who ventured out on their own but most people that I knew chose to hang out and enjoy the beach/pool. I personally enjoyed the pool area because the beach was kind of rocky. The pool was salt water so I figured it was close enough
I do recall seeing some people snorkel from the beach.
Feel free to e-mail me with any more questions.