There are a number of popular makes but before going there, you might be best served by trying to figure out where you'd put it in a boat of that size. Among the considerations would be weight distribution, routing of duct work, void space in terms of whether you'd have space for one compact unit or one which can separate the condensor and compressor components.
It's also likely you'd need to upgrade the electric service onboard.
This is not to say I wouldn't suggest a permanent unit which is quite do-able and you already recognize it can require some careful planning. Going through the effort to do it right will pay off in ease and comfort later.
Among the better units is the Marine Air smallest I believe is 6000 BTU which might suffice for you.