I will sometimes use a two conductor untinned round cable such as an SO type wire for the external connections to the solar panel but most often a tinned marine grade wire.. Inside these terminals the tinned wire is not as critical as with open style crimps. I almost always use 10/2 or 8/2 wire from the panel to deck if using MC4's. Once through a deck gland and into the vessel you can switch to a larger GA wire to minimize voltage drop.
Here I have stripped back the outer jacket of the wire exposing the inner 10GA conductors.