Hit the starter a few times with a wrench while someone tries to start the boat. If it starts, you need a new starter or get yours rebuilt. If it doesn't, find a voltmeter and make sure all the correct signals are getting to the starter. (+12.5V constant on the fat red wire, +12.5V on the skinny wire when someone turns the key to the start position)
I bet there are local electrical shops in Italy that can rebuild it. You could also take the starter apart and clean the rust out and probably limp along until you can get a new one.