Short version: don’t solder your wiring.
Brief discussion:
Wire comes in two forms, solid and stranded. Solid wire is cheaper and smaller for the same conductivity, which it is why it is used in household wiring. Stranded wire is more flexible and preferred for vehicle applications.
In the boating world, the preferred wire joint method is heat shrink crimp connectors. The crimp provides the strength and conductivity, and the heat shrink protects against moisture and the link. Crimps are easy with the correct tool, such as this Klein tools crimper.
For a detailed description of wire termination, see Marine Wire Termination. The key point is this:
“Solder wicking causes the multi-strand conductors, which have high flexibility and stability against vibration, to become, effectively, solid which degrades both the performance characteristics mentioned.”
Tom Michielutti at AMP
In order words, by soldering you’re changing that flexible multi strand wire you paid more for into a more fragile solid wire that is more likely to break due to movement.