Low speed? Get a "paddlewheel" type speed transducer from a local or online boating / yachting supply store (chandlery) or make your own. Little paddle wheel sits half in, half out of a cup (which is embedded in hull of boat). As boat moves, paddlewheel turns. Affixed to the paddlewheel is a magnet which trips a reed switch or hall effect sensor each revolution sending you a pulse.