Cheap Window Unit, and some plywood
If you really want AC without leaving the sea cock open, get a cheap small A/C unit (Home Depot, less that $100 or so) and a piece of plywood. Fit the plywood into the companion way, and the cut the AC into the plywood.Depending on boat size, this may or may not keep the boat cold, but they let you close the through-hull and it is a lot cheaper than those carry-on marine units.Several people around here have them, some in place of AC, some because they don't like to leave the through-hulls open even for a short time when away from the boat.