Did it last March,doing it again this March--Yeah,a drysuit is the way to go. Most everyone in our group had one,except one girl. We had one boat flip twice,two girls swam-one had a drysuit and one had drytop/pants combo. One girl in a drysuit was shivering a lot(but only for a few minutes after she got outta the water).The other girl in the combo outfit was VERY cold for quite awhile and started getting pretty miserable. She was gettin the shakes,even a half hour after her swim,we had to stop and find sun for her to sit in so that she could warm her body back up. Remember,its a big canyon,lots of shade--and if you just had a swim...you know the rest. Yeah,definitely recommend the drysuit-that shits cold.
I recommend the Kokatat Supernova--somewhat affordable and plenty dry. I finally got to break mine in on the Middle Fork and i didn't even get a drop inside the neoprene neck. I was very satisfied with it. You can also find used ones.