Are there any stickers on your alternator that has any info like amps, part number etc.?
What year is the boat?
Sort of hard to tell what you may have exactly as Volvo offered two amp sizes in the repair manual but the manual didn't say what amp fits what engine model, etc..
Volvo has a 51 and 65 amp alternator within the ford MD manual....Looking up the early Volvo part numbers seems to show the 51 amp size or close to it at 55A as the most common....The high output would be 65A.....All the aftermarket alternators are all over the amp size range from 51 amps to 105+ amps depending on where you search.