Nassau is roughly about the same latitude as Miami, so temps are comparable in the winter. Just do a bit of research to see what the average high temperatures are and the average water temperature, so you will be mentally prepared. I remember we took a cruise (on a big boat) one December to the Bahamas. The temps were about 70 and there was a cool rain. The ladies in the straw market in Nassau were wearing hoodies and/or sweatshirts!