For snorkeling in PR, try the eastern side of the island. Very peaceful, few crowds, beaches are more conducive for snorkeling. You may want to consider Culebra.
Turks and Caicos is a good choice, especially near Grace Bay (very calm snorkeling), but also on the NW point of the island, near their National Park, where the reef, and more varieties of underwater wildlife reside.
Cancun can be hit or miss, due to high winds and rough water, which seem to be more often than not. Better to go further south towards Riviera, or over to Cozumel.
St. John maybe has the most variety of all.
Bahamas: the eastern string of islands, Eleuthera.