Best snorkeling we ever had, and we have done quite a bit -- Eastern, Western, Southern routes -- was with Captain Nautica in St. Thomas. Small groups, great crews, beautiful spots for snorkeling with TONS of varieties of fish, etc. and even nice boat snacks. Booked them through RCI and can't wait for March to go again. Do NOT do Pinnell Island on St. Maarten with RCI unless you like about 60 people in your group, all kicking and thrashing to see the fish that show up when the fish food is put in the water. You'll see more of people's hind ends and fins in your face than you will the fish. Cozumel also has great snorkeling, but still, Captain Nautica was our best experience ever.