December Snorkeling?

[Golden Rose]

We're doing a cruise during the Christmas holidays, near the end of December, that goes to Key West and Castaway Cay. One member of our party really, really loves to snorkel, and is trying to figure out if she will get to this cruise. I guess this is really a two part question.

Is it going to be too cold to get in the water?

Is there anywhere really good to snorkel out of/off Key West or Castaway Cay? (She did snorkel just off the cabana beach last time, but she wasn't particularly impressed with what she could see there.) Is there an excursion she should definitely book or a place she should try to go?