I have an emergency, 8,000-watt generator at home. It's powered by a 5-HP, air cooled engine. When we were without power last summer I was able to run a 10,000 BTU air conditioner, lights and a TV with no problem at all. That 10,000 AC unit cooled 1,450 square feet in no time at all. There, I believe it is quite feasible to utilize an automotive air conditioning system on a sailboat and not consume very much HP from the engine while running. I believe the only reason it has not been done at this point is no one would want to pay for the installation of this type of system that would not likely be used very often.
Now, if the boat only has a 10 to 15 HP diesel engine, 5-HP could constitute a loss of 33 to 50 percent of the boat's available power. Keep in mind, though, that the HP rating of the boat's engine is usually at 2/3 to 3/4 of the engine's maximum RPM (sometimes higher), therefore a 5-HP drop at lower RPM would really be significant.
Boats with larger engines, such as the 55-HP Perkins, 30-HP Atomic-4, etc..., could easily handle a relatively large, automotive air conditioning system and suffer very little power loss. At one time in my life I owned a VW Micro-Bus that had a air conditioner. The van had a 4-cylinder, 36-HP engine. When there were 4 adults in the van, the air conditioner had to be shut down in order to maintain speed when pulling up fairly steep hills. On flat roadways it ran just fine with the AC running full blast. When my 33-Morgan O.I. is underway, the A4 runs at just 1,800 RPM to push it along comfortably at 6-knots. I sincerely believe that the engine would easily handle an automotive AC unit, but I'm too old to undertake the project.