Have had good luck using a piece (1’ x 2’) of ShamWow fabric laid at the bottom of the boat. Used it on a couple river trips (not white water, I’m in Nebraska). It worked good for soaking up the mud and water from my sandals as well as give the bottom of the boat a little extra protection from my shoe heels. At the end of the day I rinsed it off and used it to wipe down the raft. It was a little more practical than a sponge that I would use in my sea kayak. Just thought I would pass this along.