One model of aircraft I test fly doesn't have AC, so we bought an ice chest with a fan and an electric water
pump that circulates cold water
through a coil and the fan blows air over this coil and blows out cooling
air. Similar to this one
GO COOL PORTABLE AIR CONDITIONER from Aircraft Spruce
Holds about 20 lbs of ice and really works well, for about 15 or 20 min, then all the ice is melted and the water warmed up.
Now for its intended purpose that is OK, cause in 20 min I'm up at altitude where it's cool anyway, but for a boat? You get a min of cooling
per lb of ice, roughly.
Energy required to freeze the ice with a vapor cycle compressor
would I think be just a little more than cooling the air directly with a vapor cycle compressor