I will second the recommendation of a liveaboard dive boat. From day one you and the other guests are together in a little floating oasis, so while it's pot-luck who your fellow travelers are, they're all there, all the time, so it's easy to get acquainted, make new friends, and identify people you'd like to dive with. Resorts offer less interaction with your fellow guests, I think.
Does ScubaBoard offer listings for people who are looking for roommates on dive trips? Like the OP, I'm a woman who sometimes travels alone, and it would be nice to have a resource to find compatible roommates to avoid the "single supplement" cost. Of course there's some risk in this, that you might get stuck with the roommate from hell....
Gale