Working on the boat is a euphemism for masturbating. Our friend would give an excuse for not hanging out with us by saying that, "he was working on the boat". Eventually we got so bothered by his "working on the boat" that we went to his house and asked his mother and she told us that he was in the bathroom. However he didn't exit the bathroom for 45 minutes. Thus he was "working on the boat".
Person 1: Hey were going over to my house, want to come?
Person 2: Sorry, I gotta work on the boat.
Person 1: Where were you man?
Person 2: Sorry, I was at my house working on the boat.
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