To fend a boat or ship is to prevent her striking against any quay, jetty, vessel or any object that may endanger her. Hence a fender is an object used to soften the blow. To ‘fend off’ is to prevent another vessel or object (quay, jetty, etc.) from striking a boat or ship. We use it more commonly in the sense of keeping something away or even in fending off an attack (even a verbal one) of some kind.