Automotive alternators lack the fire (spark) screens and typically lack adequate
cooling for marine use. They are designed to be cooled at idle by their fan, but to be cooled by the air rushing under the
hood at operating speeds. So in a marine
installation you may need a dual fan, or a blower or ducting to add
cooling during prolonged operation. With a diesel and adequate cooling, "automotive" should be just fine, although diesel vapor CAN ignite, so spark arresting is never a bad idea.
"Most alternators can be rebuilt " Yes, but rebuilding is how you limp home. In the alternator
trade, "rebuilt" means "fixed it". What you really want is a
remanufactured alternator, which means they tore it down to the bare frame and actually tested each component to make sure it is
within spec for a new part--not just operational and worn. Don't ask me how many years it took before I learned that.<G>