There's a reason AC is not widely available in BVI charter boats. We haven't been to the BVI in May, but we were in the USVI (next door) last April and have been to the BVI probably 8 times in the past dozen years. Occasionally there would be a warm night, but never did we think we'd need AC. All of the boats down there should have fans to keep the air moving and that has been enough for us on the warmest nights.
Another issue is that you would have to run an engine to power the AC. That's not what you or your neighbors in a mooring field need. There isn't much in the way of slips to tie up for shore power once you leave Tortola, so you can expect to spend your nights on moorings for the most part, with the possibility of anchoring in some places (like Benures Bay (Norman Island).