Might just be the way I sail or something, but I've never had a problem with leeches as far as my boat is concerned. As long as I kept sailing, even at the slowest speed, they never seemed to attack it. (Skeletal and Draugr archers, on the other hand....) And as soon as I got out of the boat, they were easy enough to deal with, since leeches seem to lose interest in your boat when your tasty viking body's not in it. It's a bit of a surprise for me to read that other people do have a problem with leeches eating their boats.