Virgin Islands are good for "long stretches with no people" beaches, for when you just want to lie there, comatose for a week. Cayman Islands are great if you want to actually do something without it being a big schlep... you can basically fall into the water anywhere in diving gear. Guadaloupe has to have the worst beaches on the planet. Jellyfish everywhere, plus all the Parisiens bring their poodles so the beaches are littered with doggy doo. If you find yourself with a cute marine biologist from Wood's Hole there's a nude beach on Martha's Vineyard that's really pretty. I personally was never there with a marine biologist, and was only 17 and had never seen a very old man naked before... which affected me emotionally, but do remember it was very picturesque.
