I will give my 2 cents on this for a couple of reasons, one being the unit I have will be shipping to you shortly, and two- I just installed a new unit on Saturday. The unit is about 12x12x12, the air supply will be coming from the right side of the unit, mid mass towards the top down 4 inches. You will need space to run a 6" insulated duct to your supply location. The unit uses the area provided for the return air and requires a hole/ return duct to supply the unit air from the conditioned space.
Our unit was installed under the v-birth, mounted ontop of a 1/2" plywood. The power was supplied from the main ac panel on the frig. Breaker. The water intake came from the supply to the head, we have a T in the line behind the head that continues to the water pump, I suggest a lone supply as this was on our marine survey to fix, the company told us a T valve just needed to be installed. The pump is wired straight to the ac units power.box and turns on when the unit is turned on.
Our unit has two supply ducts and I am planning on adding two more to our setup as the air at the registers is really strong. We also moved from a 12 to a 16000 btu unit.
A company makes a thing? To add to the water discharge line that removes any condensation, this may be a good way of mounting the unit on it's side somewhere.
I found that aquaair has the registers needed and lowes has a 25' 6" insulated duct for 24.00. I have been up for the last two nights looking for the design I want to put into our boat, for now the two supply ducts work and cool the boat fast, maybee I am crazy but I want some smaller 3" round vents that run in the rear of the boat as well, but if the companion way hatches get opened to get in and out it may be better as is.
I would make a cardboard box about the size of the unit and go buy a cheap dryer vent to really see where you want to install the unit, the water pump was about 3rd away from the unit.
Try and stay away from the bilge and the engine, the fumes will fill the cabin with fumes and co from the motor.
I hope this helps,