We were there in November several years ago and were fine without A/C. I won't tell you I didn't miss it - I am a big fan of A/C - but the rooms were well-ventilated and all had ceiling fans. We got used to it. It was all part of the atmosphere and the charm of the place. I don't think it is going to be a lot different in March.
If you like the diving in St. Vincent, I think you'll like St. Lucia. From what I understand, the diving is very similar. The boat rides from Anse Chastenet are generally very short. Many are 5 minutes or less and I think the longest (down to the Lesleen M wreck) was no more than 15. One of the best dives in St. Lucia is the house reef at Anse Chastenet, which is a great shore dive. The key to the diving in St. Lucia is that there are lots of little critters and lots of colorful sponges and corals. Big animals (turtles, barracuda, sharks, jacks) are not plentiful. If that sounds appealing, I think you would like it. It would be a great place for your wife to get certified. The dives are generally not too challenging and the dive operation there is first-rate.
I liked the diving in Little Cayman better. The water is clearer and there is more variety. Little Cayman is the best diving I've done so far. The resorts there are not as nice as Anse Chastenet and there is really nothing to do topside. That's not an issue for us, but it is for some people. The best diving (Bloody Bay Wall and neighboring sites) is on the opposite side of the island from the resorts, so boat rides are longer, at least for the first dive of the day. There are also shallower dive sites, usually used for the third dive of the day, on the south side where the resorts are. My wife got certified in LC.
If you are going for an all-out dive vacation, LC is hard to beat. If you want a really good combination of a fabulous resort (really romantic, by the way), lots of stuff to do topside, a beautiful island, and some good diving, Anse Chastenet is hard to beat.