Worth looking up ABYC E-11 that covers AC & DC Electrical reqm'ts including wire / insulation types etc.
Also as JDC recommended CM's Marine How To articles are very informative
Here's one on battery cables which I'm guessing should be applicable if you are considering welding cable. Also don't assume solder connections are "better" - CM has articles on terminations that debunks that myth as well
https://marinehowto.com/making-your-own-battery-cables/
All good information but not always applicable to the situation. Tied up at the dock near sources of wire, crimpers, expert advice (sometimes costly) yes it makes sense to follow all the rules that you can. Out in the boonies miles from everything, we sometimes have to make do. For instance crimping a terminal on a cable or wire can also be done with a vice grip. I wouldn't do that if it was for an aircraft, but a boat is a little different, and if you are just repairing the wiring to the head, it is not quite as critical.
It takes a little testing and skill to get crimping right with a vice grip.
They may not recommend soldering connections, but I always solder my crimped connections. I was originally trained by the USAF to repair aircraft Radar sets. Over the many years since, I have had many other positions involving electronics repair. I have seen a lot of so called professional crimps fail. When I crimp a terminal. I first tin the wire and use
solder paste inside the crimp. (Of course tinned stranded wire is best because over time it holds up better to corrosion and vibration) then crimp the wire firmly but not so tight as to crush the wire strands. Just a firm physical connection. Then I solder the wire and the crimp by flowing the solder into the crimp until I can see it has fully flowed around the wire and the crimp. I then clean the connection with alcohol. Then as an additional repair I will have put shrink wrap over the connection area. Or lacking that, wrap several turns of electrical tape around the connection. Not finished yet, I then use nylon lacing tape around the taped connection with a restrictor knot, as electrical tape over time tends to unwind.
Sometimes soldering crimps can't be done if the metal used for the terminal is not easily soldered, and then I can only crimp it. The right sized wire and
terminal for the application is also important, but that is a different issue.