You absolutely want to get a sealed marine unit.
I shopped around and finally bought one from these folks last year:
Pearland Alternator Company Inc.
1221 N.Main St.
Pearland Texas 77581
TOLL FREE NUMBER
The sold me a new alternator for my X-Star for about $140 or so (can't remember the exact price). They even helped me pick out a model that put out 20 extra amps over the stocker 51 amp. Same brand, same mounting position, just dropped it right in.
I was able to buy a brand new model from these guys for what it would have cost to rebuild mine.
They shipped it out very quickly and I got it in about 2-3 days.
Have your brand of alternator and model number available when you call and they can help you out very quickly.
P.S. You will have to swap the drive pulley over from your old unit.