Just curious, why did he give you a starter? Of all things when buying a boat a starter is not one of the things I would expect to be given.
Well, when I was younger and was off roading with a truck with an automatic transmission (no push start possible), I always carried a starter in case it died.
I suppose a boat is the same way of thinking.
With a bad alternator the engine can run a good 30 minutes if careful with the accessories, but if the starter dies, the boat won't move.