Check all connections very well. The marine environment is really nasty for corrosion. Not just rust, but the electrical terminals seem to get nasty quickly. Also, on some of the MCs a bit newer than what you have, maybe mid 90s were famous for the ground wire breaking off the back of the alt. I was reading low voltage earlier this spring and thought I had a weak alt. Went to check it and the ground wire broke off in my hand.
Also... some on here believe that rebuilding is better than new. Search for it if you want to find the debate. I was going to rebuild because it was cheaper, faster and local. Also I was going to "the little guy" which makes you feel pretty good too. Not that I don't love SkiDIM.
Also, just to be safe, if you are looking to rebuild or new, remember these marine alts are not interchangeable with auto alts. Use an auto alt and your boat may go boom.
1996 Prostar 205 LT-1