There are several ways. For my 22's and 25, I have used an "Arctic King" air conditioner, 5,000 BTU set into the forward opening cabin window. By using mylar bubble wrap (Lowes) covering the windows and the shades, I have kept the cabin of a 25 at 80* in ambient temp of 105*. These will run off a EU 1000 generator. I either put the generator on a up side down milk crate on the swim step, or on the current 25, I had an EU 2200 mounted over the splash well permanently.
The Tom Cat I put a Coleman Polar Cub, Mach 8 9200 BTU. I believe that this is the best unit for the C Dory 22, 25 and 255. John of Cat O Mine, likes his cabin like a walk in freezer so he has 15,000 BTU (?)>. The EU 2200 will run the Coleman 9200 BTU unit. I would put an "Easy Start", slow startup unit on the AC unit. Then the generator can be run on eco mode. If you don't have the slow start computerized unit, the generator has to run at full throttle all of the time. This uses more fuel, and makes more racket. You can run the Honda generator off a 6 gallon remote tank, as long as the tank is close to the level of the generator.
The raw water cooled units take up a lot of space inside of the boat, especially with ductworks, raw water pump and compressor/evaporator unit. They cannot be run on a trailer. Many of us "Boater Home' in camp grounds, and the RV AC unit works very well. The RV units last at least 10 to 15 years, if cared for well.
Keep cool. The only reason I don't have the RV AC on the current 25, is I like to carry the dinghy on the roof, and it is harder with the RV AC. Also I have much of the space in the "Buldge" inside of the cabin ceiling with the camper canvas and life jackets, so not much room for the interior air handling unit.
You can see my photo album to see how I modified the roof to strengthen it for the AC units. I realize that some just put it on there. I believe that if you do it properly, there will be no issues down the line.
Please ask questions if necessary.