I worked on tall ships as a teenager, and went on to work in the marine industry and to earn my 1600 USCG Masters license and AB unlimited. I crawled straight up that hausepipe you're talking about with a knife between my teeth and a bloody glint in my eye. I can tell you that working on boats - both physically onboard them and also mechanically - is fantastic, and deeply shitty, and wonderful, and amazingly fucked up. Rewarding, and frustrating.
That caveat is: the size you're looking to work on would mostly be associated with a small boatyard for recreational vessels. A place with 1-2 lifts, maybe up to 50 tonnes. At that boatyard, you will find mostly sailboats and pleasure craft, and a few working boats (lobster, crab, long line, just depends where you're geographically located).
The people who own the working boats won't really need your skills. They either fix the problem themselves, or they get the tech reps and/or the local "small" engine repair tech out to repair the whatsit. They'd be unlikely to hire someone like you're describing, because the person maintaining the engines is by default maintaining the power source that powers all the equipment that keeps people from drowning in their own racks.
The people who own the sailboats and small power boats would be more interested in a jack-of-all independent contractor, like you're describing as your ideal. These people are uniformly (as far as I can tell, and I'm biased) pretty fucking crazy, and only sometimes fucking rich. They will not want to pay you. They'll be even less inclined to pay you a far wage. You will have to dun them. You'd also be willingly tying yourself to a
deeply boom and bust economy.
If you're willing to commit yourself more towards the mechanical side of the house, rather than the deckhand-ish stuff with repairing/scraping/painting, you should look into the businesses that service the yards, and are patronized by the working boats. They will be steadier through economic fluctuations, and you'll be an insider, instead of an outsider. That small engine course sounds like a great idea; and while you're taking it, imagine doing whatever your doing wedged into a tiny space, possibly upside down or ass-up, or ribs-are-being-crushed over an engine, while being tossed around. If that brings a grin to your face, charge forward, friend! It sure as hell brings a grin to mine.
For reference, this is me in 2010 when I was still skinny, working on the 'roomy' side of the engine. I look like that because I was actually stuck. While slithering down, I managed to get my torso between the pipe and the bulkhead. Turns out, I was entirely unable to get my torso back out of that hole. Gravity was against me. Things compress different going up, instead of down? I have no idea. It just. did. not. fit. My shipmates had to remove the pipe.