People have all sorts of different tolerances for different conditions.
My medium temperature gear is hydroskin, both top (zipper closure in case I'm overheating) and pants. They are nice and fuzzy and comfortable. I also wear the NRS neoprene river shoes. I wear shorts on the outside of the hydroskin pants to protect them and have splash gear handy in case it gets colder.
From your description of the river, you aren't going to spend a lot of time wet. I'm not a big fan of farmer johns or a full wetsuit in dry conditions. They need to be wet to be effective.