Re: replacement batteries
You can almost certainly go with something other than AGMs. Flooded lead acid deep cycle batteries are probably the best deal out there. You will need to make sure your battery charger is set to charge "regular" batts vs. the AGMs though. You'll find some good threads here on some good deals on deep cycle flooded batts, such as the ones from Sam's Club. I buy mine at Costco.
Unlikely that the bad batt is causing the eaten shaft zinc.
2000 Beneteau 331
Northern Chesapeake Bay
Ambition is a poor excuse for not having enough sense to be lazy ~ Steven Wright