A/C on the high seas
You can always add a diesel generator.. You would need the space and it would cost alot to add (Ballpark $7-10k).You might be able to do it cheaper. But unless you had a good sized boat finding a good spot for it would be a hassle and the plumbing and necessary installation would not be trivial.
I guess you could do "something" with a diesel auxiliary engine, but it would put more hours on it than I wanted. There might be some "unique" systems out there that could run off a compressor on the engine. (They do have engine driven refrigeration systems, but they usually only run a few hours a day and freeze a cold plate) But the complexity is more than I would want on board.
If its only a temporary/once in a while thing, then I would get a honda gas generator and a smaller AC unit and just be careful with the gas and fumes.....(BTW we carry gas for our outboards anyway, so its not that big of a deal)