You want to put a good/effective air conditioner on your boat for cheap? Buy a small 5k BTW window unit from Home depot and stick it in your companion way door. All you need to make it fit is a new door you'll have to make from pressure treated ply wood, some handles to make lifting out the door easy, and some weather stripping to seal up the cracks. I put one together in a weekend and it cost me less then $150. It works great (90 degrees outside and 65 degrees inside) and the unit uses like no power. I can run the thing off of my batteries for 2 days straight. Plus you don't need to fiddle with any plumbing, thru hulls, or sea cocks. Its just a cheap, simple, and effective way to cool your boat if you need to sleep in it for a night or two. Also, its risk free because if it doesn't work out you just throw away the door and move on.
I will say that you don't need an a/c most of the time. I used mine a total of once this entire summer because it doesn't get hot enough on the water to warrant its use. When it does get hot the breeze cools you down or you can jump in the water that's all around you.