while I've never done a wedding on a caribbean island, I've seen several of them done before at these resorts.
Saphire Beach Resort in St Thomas has a nice area. they have a peninsula that sticks out from the water some that provides a walkway out to the ocean for the wedding ceremony. it was done very nicely. guest can easily stay at the resort. btw.. they have an onsite dive boat, which is always a plus (wedding or not).
Couples Resort in Ocho Rios and Negril both did them. They were nice but I was least impressed by this resort that did weddings. They seemed to do the exact same thing for the SEVERAL weddings that we witnessed while lounging in nearby beach chair. Most of these weddings were the wedding couple only with no guests. This resort is a no-children resort, so it will limit who stays there. It's also not cheap either with might limit some of your guests. Both of these resorts are also a couple hours drive from the airport which again is limiting. However from a diving related view, both resorts have dive boats on site though the diving is much better at the Negril resort. (of course we all know that Jamacia is not a world famous for diving, but the diving in Negril wasn't that bad).
Aruba Wyndham prob had the nicest setup I saw of all weddings. They setup an archway on the beach and lots of white beach chairs. The wedding I saw was towards sunet and had a beautiful setting. Aftewards the resort had a reception area set up on one of the outide balconies and served a reception dinner. It was pretty nice. This is a very nice resort also with great restaurants. (diving report, several local operators and decent diving right off the coast).
Other locations: I've also seen some great beach weddings at other US locations in the Florida panhandle in small little beach towns. you can't beat these locations.
As for the Bahamas, I've been there a couple times but never seen a wedding there. My vote would be for the "out islands" and not Nassau or Freeport. For some reason Nassau and Freeport, which are the major airport, cruiseboat, and tourist destinations, the people on that island just don't seem that friendly as the out-islands. In fact, most of them not only are not friendly, but are rude or un-grateful and seem like it's an insult to have to serve "us toursits". Yes I did see some friendly people there, but this was the exception and not the rule.
Hopefully someone else can reply with some other options so you have more choices.