December could be summertime hot, or it could be pretty darn chilly......it's a bit of a crap shoot. You are almost guaranteed that the early morning will be noticeably cooler than the afternoon, when the sun is at it's strongest.
That said, we've been to DC in January and they have things in place to help keep guests comfy in the cooler weather. The wet suits are designed to keep you warm when you're in the water, so on a cold day, go with the full-length version. Some water areas (specifically the Wind Away River and Freshwater Oasis) are bath-water warm, and good places to spent time on a colder day. There will be large heaters around the outdoor dining area to warm up guests when it's time to eat.
The dolphin pool is kept consistently cooler for the health and comfort of the animals. That water will be the same level of cool regardless of what time of year you visit. It's the air temperature that will getcha, not the temperature of the water.