Go someplace warmer if there are actual beginners on the trip. Excessively cold water is not a good thing if someone swims, one reason I feel off season boating is best left to people who have lots of experience.
Some section of the daily section may be a good option, flat water from Dewey bridge to Hittle bottom, or some class 2-3 from Hittle to take out beach. May still be a chilly swim, but warmer then upper-c at least.
" I wish I were a headlight, on a North bound train..."