Well, consider this. The water coming out through the turbines of the Glenn Canyon Dam from the depths of Lake Powell is only 48 degrees at Lees Ferry. Pretty cold for swimming. Also consider the fact that in small, oar powered craft your feet are always in cold water, and that every 20 to 30 minutes you ride through rapids rough enough to either spray you or dunk you. And you are being sprayed and soaked with what soon feels like freezing water.
Sunny days are fun.
The rapids cool you down, but if it's a rainy day you may never completely thaw out from your last dunking when it's time for another. If you are unprepared you can be in for a very miserable ride. After we had finished a hearty breakfast our bags were loaded on separate vans, each van pulling a dory. But OARS had used collapsible rafts, while Dories uses rigid, double ended row boats. Each dory must have its own towing vehicle.