Wow! A long wire run and starting a diesel to boot. Definitely try to
go with a larger wire size.

In the US wire size is specified in gauge. Unless actually in some
electronics or electrical industry most people just go by gauge size
from past experience or look things up when they actually need to.
Including me. If you want the actual diameter of a specific wire gauge
you should be able to find it on the internet. You would also find data
for spec'ing out what wire size to use for a given distance for a given
electric load.

Your boat sounds like it has an interesting machinery set up.

Earl