IMHO, Rum Point is too far and would be a LOT of your available time. Besides, it's really just a tourist trap. Tim, read the reviews before you do that.
Smiths Cove is a great little beach but quite small with no facilities. Spotts is beautiful, but perhaps again too basic for many / most. That's a great snorkel place though! If you want chairs, umbrellas, and facilities, thee are several large beach bars on SMB that cater to cruisers.
Best to do is look at Trip Advisor and get lots of recommendations and reviews.