Last time in St. Thomas, we went to sapphire beach. We just took a cab. It was very uncrowded, we usually go to Coki beach, which has an eclectic crowd, but I thought the snorkeling was better (although Penny disagrees)
In Nassau we walked over to Junkanoo beach. It was a very short walk, and it's right near the area known as fish fry, where a lot of the ethnic restaurants are located...as well as Dunkin Donuts.
As Dave said... San Juan depends on your time frame. I've never really been there long enough to venture out of down town, It's really just a big loop, but the views are beautiful and you can walk down by the forts and the old gate and back around, Not really any beaches in close proximity for the time allowed there.
Never been to St. Kitts....I' m going on 9 months with out a cruise.....boo