I can see where boat repair skills would be in demand, but I wonder how the locals would see it if you pulled into port and announced you were doing engine repairs. You're competing with local guys who're making their living doing the same thing.
First, you could find a shop that has a large backlog and see if they're willing to hire you on. That eliminates the whole competition question. You may even improve their reputation if they have an image of a shop with poor service.
Second, if you pull into an area that has shops of poor reputation, then you're doing the local community a favor, and to hell with the hurt feelings of the local shops.
Lastly, if you pull into an area that has reputable shops, but they are heavily backlogged and still not interested in hiring you, then you're really not hurting anyone. Odds are, as a one-man shop, you wouldn't be siphoning off enough business to hurt anyone.
Your question really applies more if you pull into a foreign port and start putting the locals out of business. In the US, competition is expected and there's nothing wrong with it.