Luis Pina Island south of Culebra is a marine sanctuary and has great diving... both snorkel and scuba. There is also a wall in 45-feet of water on the south side of Culebra that is spectacular. The dive shops know it. Again, good for snorkel and scuba.
I can recommend Taz with Aquatic Adventures... located at the east end of the channel with the Draw Bridge in Dewey.
If memory serves it's called Mamacitas, or The Dingy Dock... not sure which. The shop is (or was) located upstairs.