The Yanmar 1GM comes equiped with a 35 amp Hitachi alternator. Automobiles no longer use these low power alternators so chances of finding a suitable replacement there are slim to none. Power in the small engine is limited so be able to properly drive the boat and run the alternator at the same time a 35 amp is about as much as the engine can do. An aftermarket replacement alternator for your engine will run you between $175 and $200. A brand new Hitachi will probably be $350+. I would take the old unit to an electric shop and have it repaired or rebuilt. The salvageable components on the original Hitachi may be of better quality than those of an aftermarket unit.