With a wire heating up as you describe, I'd say the problem is in the wire or connections. There shouldn't be that much resistance in properly sized wire. That's what I would think needs to be replaced.
Disconnect pump from switch. Get a bunch of wire same size as in there. Temporarily run it from switch to pump and turn on switch and activate pump. If that wire gets hot, the problem is somewhere else (bad pump, float switch, etc?). If it doesn't get hot you can surmise that the problem is in the wire, etc. you might have a short due to abrasion in the wire that you can't see. It only might happen in rare occasions. But a short can start a fire and fires do not belong on a boat.