It can depend on the day, too. It can be warm or cool at that time of year.
Personally I'm not "inspired" to swim when the air temp isn't warm, but there are those (especially kids) who won't care about that as long as the water is warm.
I am going 12/16 - 12/25 and will be packing a swim suit. Will I use it? Maybe one or two days, or maybe not. But it doesn't take up much room, and I want to be prepared.