As far as keeping feet dry, I've had good luck putting my feet up on the little ring that surrounds the middle of the boat (below where you put cameras, etc. to keep them dry). This keeps my shoes out of the pooled water at the bottom of the boat (there is a drain but it doesn't drain the water immediately), and any water that splashes onto the outside of the shoe from the river dries off pretty quickly, and my socks stay dry.
For the rest of your body, ponchos really do work well except for lower legs and feet.