Boats arent perfect. Zombies are human bodies, and human bodies are pretty buoyant. Not sure how long it would take them to get waterlogged and sink, but at least staying in water 10 feet deep or more could work. I dont think they would be especially effective while floating, and if you are 10 feet above the bottom of the water, they cant reach up and grab you or your boat. Biggest issue might be floaters caught in a current going past you and latching onto your ride. Or reaching a section of river/lake bottom shallow enough for them to grab you.