We've cruised in December a few times. It will be plenty warm, probably mid-80's and humid.
Castaway Cay has the potential to be chilly (70's), but for us, it's always been warm. The only difference from cruising in May is that it will be a bit cooler, and the days will be shorter. On deck at night, it should still be warm. I don't think I ever needed a sweatshirt.
The water temperatures on the islands should be fine. I remember in December of 2001, it was raining in the morning on St. Maarten, but we spent a good two hours in the ocean at Dawn Beach. We got tired before we got cold, and that was with no sun. Castaway Cay was a bit cooler, but we still spent plenty of time in the water.
I believe the pools are heated, but they never seem so to me. Probably because the air is really warm. I've always thought the pools were chilly, no matter what time of year we've cruised, but I usually only jump in them to cool off while lying on a deck chair. I don't spend an awful lot of time in them.
