Vincent54 has a thread called Brac Trip Report, I think, from May, where he spoke well of the shore diving. Grand Cayman has some shore diving. I don't recall about Little Cayman either way on shore diving. In the U.S. Virgin Islands, St. Croix has some shore diving - Cane Bay Dive Resort has some shore diving (I've read the swim outs can be substantial if memory serves), and there are a few threads about St. Croix on the forum. I don't know what your odds of landing nitrox are with any of those, but thought I'd mention them.
As another poster bought up, Curacao may be your best bet, but the others may deserve a look, too. I'm on my iPad, so looking up stuff and hyperlinking isn't all that practical right now. I figure if you know what to Search for, it'll be easier to find...
In another thread, someone posted about good shore diving in Broward Co., Fl, though I don't know much about that (I posted back asking about it). Sometimes a good tropical vacation doesn't require leaving the U.S.
Richard.