I have a 1983 Hunter 34 and installed mermaid m16 unit 16,000btu
It was the smallest size unit I found that produces the btu's I was looking for. Only thing i would suggest is get the largest unit you can.due Boats are not insulated and Ac units have a hard time keeping cabins cool in direct sun in the really hot summer months.
Mine does well however on extremely hot days in New Jersey in mid August the unit struggles. Anything that you can do to reduced heat gain within the cabin helps. Examples hatch covers, curtains, and insulation can really make a difference.
If you order a mermaid unit call them directly there sales rep will work with you on price and they can make up a customized package for you. I saved approximately $150 vs buying from a distribution center. Big plus was they packaged exactly what I wanted as far as discharge vent qtys.
If I were to do install over I may have gone with larger unit however I had space and power supply limitations.