I believe that the water temp for the dolphins, stingrays, and the tropical fish is regulated. If it wasn't, it cause problems for the creatures. Dolphins do better in cooler water, while tropical fish can't take it when the water gets really cold. I'm not sure how much tempature regulation is done to the water where they aren't maintaining fish and other creatures, though.
I have been to DC when it's cold out (high in the 50s), and once you got into the stingray pool and got accustomed to the water, you didn't want to get out because the air felt so much colder. That seemed to be the warmest water. The lazy river water was cooler than that, and the dolphin water was the coldest.
As for how cold it will be outside... well, that varies a lot. Have a look at the
historical temperatures to see what it has been in past Decembers. Last year, the records show Dec 7 with high temp of 65 and a low of 40. 2002 you have 72/47 (but Dec 31 of that year you see a high of 80... go figure!). 2001 is 80/64. Florida weather, more than any other state, is impacted by lots of outside forces because it is a peninsula, so you can only predict it in extremely general terms!
