This was our second time back, despite 39 degree air temps the first time. Keep in mind the water stays at 72 degrees year-round, so staying in the water as long as possible is a must when its cold. The math is easy - its warmer in the water than out!
This past December (2013), it was in the 50's in the early morning - very comfortable. The boats are equipped to keep you warm, and they (Bird's) serve hot coffee and hot chocolate on an enclosed boat. By the end of the trip, it was very comfortable out of the water, as the air temps were rising naturally.
Make sure you get up for the early 'Bird's" trip - they put you on the water and near the manatees just after sunrise. Your the first one's there. It's an awesome experience.
Also Don't miss the Three Sisters spring - its a must see. The visibility is world class and the manatees love to hang out when the water in the Gulf is cold. Keep in mind the traffic can be busy during the Christmas vacation. Try and schedule a week-day.
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