Depends where you reside. When we go in January we are the crazies in flip flops, tanks and sundresses. and of course we swim at the pool. But we live in very Northern Canada so January weather in California is like our summer weather.
It's no exaggeration that Southern California in December can go in a number of directions, temperature-wise. I've been to DLR when it was 90 degrees in the daytime, or 80, or 75. It's also been 50 degrees in the daytime, and breezy. It could be warm and really gusty. Sometimes it could be drizzly and gloomy. It could also be pouring heavy rain. It could be 30 degrees at night, and 40 degrees by dusk. I have experienced all of these scenarios in December -- in and out of Disneyland Resort -- and I never know what kind of mood Mother Nature will be in each year.
More than likely, though, the chances will be good that it will be warm enough for you to swim on at least one day of your trip.