I would find the oldest and crankiest auto mechanic you can find. If they use phrases like "don't need those damn fangled computers" you know you found the right guy. Preferably missing most of there teeth, and may even do the work for cheap just to get to work on an old simple motor! Names like Cletus, and Jeb are good indicators that they may work out well.
In all seriousness, what do you need for the annual service? They are basically 1950's tractor motors nothing out there simpler than these motors. I think the best thing is to do it yourself as you will get to know the motor and will not be intimidated if there are issues with it when you need it out in the ocean. You should be able to pull the carburetor apart yourself and set the points as well just in case you need to do it in order to get back in someday.