If you are staying in North Side/Rum Point or East End and the weather is lousy for snorkeling (happens, especially in winter), try heading over to Bodden Town (click for guide) or other south or west facing shoreline areas like Smith Cove and Eden Rock in George Town or Seven Mile Beach (Governor’s Beach or Cemetery Beach sections).
If you are staying on Seven Mile Beach and it is too rough, there is a slight chance that conditions along the North Side may be better.
Good Luck!