I am planning on snorkeling there this summer. Dzul-Ha is about 3 miles south of town. You do not need your own gear, as there is a little dive shop on the premises. As far as how far out you need to go, I would say about 50 Yards. You're in about 15 - 20 ft of water.
Snorkeling is pretty good there.
If you take a cab, they will know where to go. Tell Joel, Manuel, and Xavier hello. They will be at the beach bar. I am planning on snorkeling there this summer. Dzul-Ha is about 3 miles south of town.