It is pretty much the same all year 'round. The water is smooth, and nort warm like in the deep Caribbean, but still swimmable. Take your Kids to Atlantis and let them see the aquariums. They will love it.
if conditions are right snorkeling at Coco Cay is great--wrecks to explore, protected area--just beautiful, lots of fish--ther nice things to do on land & I haveheard there's even more than in the past. Other Snorkeling in Bahamas can be rough--we did a trip off a boat & found it a bit tough going---