My kids swam every day of our trip in January 2011. Mind you, we're from Saskatchewan so we thought it was balmy while Californians were wearing hats and gloves . I see you're from North Dakota so I'd say unless the weather is very bad they'll be able to swim.
As a SoCal resident, there's no way I'd be caught swimming in January barring some unusual heat wave, BUT, that's because I'm a local. If you're from somewhere cold, then maybe 50 or 60 feels swimmable lol.
We were in DL 12/29/11 - 1/6/12. It was in the low 80's during the day and definitely warm enough to swim. Actually I think that Christmas trip was warmer than our Memorial Day trip in 2011.
There's pretty much never a bad time to go swimming in OC. Especially if you're from a colder climate, i.e. the east coast. I don't know for sure if the pools are heated, but I don't think that would be a problem anyway.
We're from Oregon and were there in mid Nov 2011 - we apparently hit a cold snap for the first part of the week and it was pretty cool BUT by the end of the week it was definitely swimmable, especially midday. My kids swim in the Pacific at the Oregon coast though - so YMMV.
We go for christmas every year and Yes, the pool at DLH is very nice. We go swimming/hot-tubbing everyday. I will be one of the tourists wearing shorts and a t-shirts in the park and smiling a lot in the pool....