Would you expect to be able to comfortably swim the last week of December in Palm Springs? I didn't know what to expect with the weather or if they heat their pools in the winter.
It's hard to say about Dec. weather. I live about 80 miles or so west of PS and today, mid-Nov. we will be over 90! PS will be almost as hot, about 88. We are experiencing a heat wave.
The sun does set early in Dec. and then you can expect PS to get cold. The sun sets about 4:30-4:45 in Dec. and with that big mountain, depending on where you stay, the shade could hit your resort hour or so earlier.
Since Dec. is season in PS I would assume that the pools will be heated (call your hotel to verify) and the whirlpools will definitely be heated.
So basically, as far as swimming goes, it'll be a toss up. Bring your suits, just in case, but don't plan on it.