Which ship are you going on? Majesty Of The Seas?
You can wait and book tours on the cruise. Despite their dire warnings about 'some tours selling out fast' I have always considered it a sales ploy in most cases. This would give you an idea of how weather will be without committing to a tour weeks ahead. And yes, just because it is too cold for you is not a valid reason for a refund.
Nassau is indeed largely touristy as far as shopping, but then how else are you going to get the t-shirts to prove you went there?
Atlantis is mighty proud of their resort and the prices reflect it. The ultimate tour has a lot of physical activity, but then you don't have to do it all. Still it is a $300 commitment. Snuba...how claustrophobic are you? That is one of the biggest issues some have with it.