Personally I wouldn't bother with a/c but make sure the charter company supplies wind scoops for the deck hatches. They are like little tents to catch the wind and direct it below - makes a world of difference and helps to keep the rain out too. BVI is pretty safe so, as long as I'm not in a slip (which, for me, is a last resort) I leave the companionway open overnight as well, to let the air circulate.
Geoff makes an important point. As a liveaboard boater I could control comforts but as a boating writer who sailed on many other boats I often found myself in hot, stuffy cabins. It's important to know what the charter boat provides.