Stranded all the way in a boat/truck.
If this is a salt water boat, then tinned wire is a must. Fresh water boat, non-tinned stranded wire will be fine. Actually wiring designed for auto use will work just fine for a fresh water boat. Honestly your truck wiring sees more water than your boat every will. Look at how the auto manufactures handle the wiring under the truck hood, and under the body. Mimic it and you wont have any problems.