snorkeling excursions from royal caribbean

want to do some good snorkeling and was wondering how long do they usually allow you in the water at the good snorkeling spots? the 2 I have to choose from says 2 sites then a beach stop. don't care about the beach, only want snorkeling.

are the royal Caribbean excursions worth it or should I just get a taxi and find a good snorkeling beach?

I would look at private snorkel tours instead of the ship sponsored tours - usually cheaper and less crowded.