We found some great snorkelling - mainly fish life at Tobago Cays on the outside of the reef. Can take the dinghy to the edge of the dinghy passage - tie up to one of the buoys on the inside of the reef. Then swim thru the dinghy passage to the outside of the reef. Need to be a strong/confident swimmer and of course watch the currents.
If the wind/waves are too strong then visibility can be a problem.

Bob