We went down WW in late November a couple years ago, and that was plenty cold for me. 41 degree F water temp; a nice day, but no sun in the gorged section. A good way to try out your cold weather boating gear (after roadside tests at the playpark). We had a swimmer who did not have a drysuit; just a drytop, splash pants, and fleece layers. He was very cold. I was trying not to flip
. A few playboaters were flipping repeatedly, hardcores!
Since then I've found that a 2mm dive hood is way better than a skullcap, to prevent "ice cream headache" after a flip in water below 40. Earplugs help too. Also, every 5 degrees of water temp is a very noticeable difference. 35 deg is way colder than 40, which is way colder than 45.
I see that the USGS gauge for Westwater has pretty detailed info about the water temperature, with the daily high and low water temp.
USGS 09163500 COLORADO RIVER NEAR COLORADO-UTAH STATE LINE
Choose "Time Series: Daily Data", and pick your days. Here is 2010-11-1 to 2011-03-1:
USGS Real-Time Water Data for USGS 09163500 COLORADO RIVER NEAR COLORADO-UTAH STATE LINE
There was a deep freeze in January; and look at how the daily high water temp was 32, as well as the daily low. I bet WW froze over (especially after looking at some Jan air temps in Moab; lows averaged 4 below for a week).