ABYC standard is to crimp and not solder joints. As mentioned the issue is flexibility and corrosion. While a ratcheting crimper is the best a good quiality regular one will work. Just tug on the joint afterwards to see if it's a good crimp. Have enough slack in the wire so the joint's not taking any strain if things shift a bit. I'd use the heat shrink that seals over plain heat shrink in a bilge bilge or external area pricey but worth it. You can fiberglass a mounting fixture to the bilge for the pump and switch. Most pumps I've seen are mounted from the side so the bottom is in the bilge.