We did the Dolphin excursion at Sea World two years ago in December. It was the coldest December morning Orlando had had in quite some time. We put on our wet suits and went into the water. The only time we were cold was before they let us in the gate to start our adventure, after that everything was great. Discovery Cove wasn't open yet, but they were in the process of training the Dolphins for DC, so we got to do a lot of extra things with them. We had a blast. Oh, and we were the first Dolphin excursion....7am if I remember correctly, before the park was even open. Just to be on the safe side, you may want to schedule that part of your day a little later in the day after the sun comes out, just in case you hit a cold day. Other than that...you should be fine.