What solder to use on basic automotive wiring?

Let's keep in mind at the modern solution to keeping the crimped connections from corroding: sealing the crimps reasonably well from moisture. This is done nowadays in cars by good seals around connectors that are exposed to moisture, both for the contact and the crimp. You can do this in car wiring by using some 'flooded' heat shrink tubing over the crimps, and avoid the problems of work hardened, solder-wicked wires breaking off.

If you do solder, then just be sure to support the wires a few inches away from the solder joint. A boat would be included; the pounding and shaking in rough waters is pretty major. Like I said, I stopped soldering in my race cars: those are rally cars that take constant pounding and shaking and vibration. You learn pretty quickly what does not work in that environment!