I agree with Jim's recommendation . Trunk Bay is beautiful even if you don't snorkel! Although beaches in St. Thomas like Megan's bay are very pretty, IMO those in St. Johns are superior.

I'm going in a Silversea cruise in March that goes to Tortola. We are planning to do the Swim with the Dolphins. I know some people frown on that but it is a lot of fun. My sister and my son did it last summer and I saw the video. It's unbelievable.

Hope you enjoy a lot of piñas coladas on your cruise!