Solar powered cooling
Boats got plenty of sunlight and a huge heat sink below em. How do we use that for cooling? Check out something from about 100 yrs ago called a Crosley "Iceyball". It used evaporation and condensation of ammonia to produce ice and it was regenerated by heating on a burner. Replace said burner with a solar concentrator and Voila, ice from the sun. However, I dont like the idea of ammonia on a small boat so....................Zeolites.
Zeolites when dry absorb water vapor like mad giving off heat. Use the heat to regenerate another zeolite container. When it absorbs water vapor, it causes cooling of the water reservoir producing ice.
AS I really dont need ice on my small boat, I wonder if this could be used to cool my boat. It would certainly de-humidify incoming air and running the air over ice it produced would cool it. Regeneration would need to be supplemented with a solar concentrator.
OK, lets go further.........after considerable checking of the patent office, I suggest, zeolite aided desalination. Two separate zeolite containers so one can regenerate the other. A salt water reservoir connected to the zeolite containers. The humidity captured is fresh water. it can be driven off the zeolite by heating and recondensed via the cooling produced by the evaporation in the salt reservoir.
Turns out that reverse Osmosis is more energy efficient than any form of distillation except we have nearly unlimited sunlight.