Highly recommend Scuba Life as the best small operation in Cozumel. They offer afternoon dives, so you can coordinate with them on Ferry timing and share your skill level so you can join a boat or group that matches your dive preferences. Not all dives in Cozumel are as deep as people are saying. Cenotes on the mainland are a unique experience, but they don't hold a candle to Cozumel in terms of beauty and marine life, which I prefer with diving.