A/C on the high seas
I''m not sure what boat you have, but if you wanted to just cool a small salon area you could get by with a ''smallish'' AC unit (6-7k BTU). That wouldn''t really cool all of my C36. But if I closed the fore and aft cabin it would probably cool the salon area ok. But the beauty of a small AC unit is that you could purchase a small gasoline powered generator (Like a honda unit). I have been told that these units will work fine for the smaller AC start-up current requirements (Thats the tough part, be sure to spec the generator to the start up current requirements of the AC unit).
The other good thing about these small generators are that they are relatively inexpensive, lightweight and are easy to use. (Not the expensive and complicated plumbing, like if you installed a permanent generator).
The downside is that you must store gasoline on board and then be carefull of the CO fumes . I would definetly have CO detectors installed. (I have heard people that leave it in a dink off the back of their anchored boat). I have also been told that they are very quiet though.
If you needed more BTU''s cooling for your boat you could add a second unit (another 7k BTU ?) and then when you are at the dock you could have the shore power easily run both. But being split up into 2 units make it easier to run one AC system at anchor.
Ok with that said I was going to get one 16.5k BTU unit for my boat. But it will be next to impossible to run at anchor without having to have one "Big A**" generator installed. Something I would never do.
You can basically forget about having Batteries run AC. The current requirements for even a small ''traditional'' AC unit is phenominal and would drain even the largest battery banks quickly.
There is an alternative and it works off your batteries...but I am not sure how well it works, but it might be worth a look. I am suspiciuos... Take a look here http://www.coolcurrents.net/
Good luck .....Good Sailing w/good anchorages.....and calm nights....;-)