The benefit of finding wood that's already saturated (something that's been sitting in the stream, for example) is that you know it'll sink, and you definitely won't have to worry about weighing it down. The only thing I would be careful about is to make sure the water's fairly clean. A stream is probably good, a drainage ditch next to a road may not be.
Welcome to the "Sticks and rocks? I'm not paying for that s#$%" club.