Nassau Bahamas would be 74 degrees which is a little cold and really will depend on how hot the outside temp is. The ship will have nice warm hot tubs though. St Thomas will be some what warmer closer to 78 - 80 degrees.
Make sure to go to Magan's Bay Beach. Take the local bus or taxi which is $8.00 per person and $4.00 entrance fee toget in the beach area. Snorkling is good there too. It is a large Horeshoe Bay cut off from wind and waves on the ocean so the water temps are higher. St Martin should be about the same but I don't know of any beaches. Have a great cruise. Ed