I'm interested in St. Croix as well. If your wife is a diver and you haven't been there yet, try Little Cayman. Wonderful diving but not a ton of other stuff to do on the island.
I'm good with nothing to do other than diving. We really enjoy Bonaire and CCV I do have LCBR and BCBR on my list, however looks like Brac Cayman Beach Resort is closed Aug 1st through October for some renovations I ready.
The other two places I'm considering are Dominica and Saba.