Depends. Fulci zombies can walk on the bottom of the body of water. IRL - a corpse will float as long as it continues to decompose. Sometimes they will make a big "burp" and then sink. But they will come back up as soon as more gasses are released during the decomposition. So if they are still a zombie and they do have coordination, they can probably do like a float and swim kind of deal. If you have a boat, just make sure the zombies floating by don't have arms.
http://www.youtube.com/watch?v=uOSN2s8FY8Q