I'll 2nd the concept that it isn't fresh water that kills alternators, but the grit and grime that make water into mud. I've had more than a handful have bearing failures (2 or 3 delcos, 3 Isuzu alternators), and two electrical failures along the way. There are shops that can rebuild alternators with better (sealed) bearings if that's what you have fail out of warranty.

I'd happily take a powder coated alternator housing. For some reason my alternators always seem to corrode more than anything else on the truck. I'm pretty sure that powdercoating won't effect cooling, but it is something to think about. Paint won't radiate as well as metal, but I'm thinking that the internals produce the heat and the fan cools them directly, not that the case is being used as a heat sync for the internals.

Someone around here must know for sure about the cooling issue.

-Tad