My aircon does not draw air from the outside, but from inside the boat. I think most boat aircons are this way. So the a/c sucking in fumes from outside is not an issue.
I would suggest this risk is a potentially flawed one. Although the actual unit may only draw air from "inside" the cabin, the air that is inside the cabin, came from the outside a few minutes ago, and unless you are an airtight boat, which is nigh impossible, even more will be drawn in as the AC does it's thing.
The AC may not suck air from the outside, but I guarantee that outside air makes up a lot of the inside air and even more so as the night falls, and convection and thermal being what they are. There are tons of places where CO can be drawn in, just think of the normal vents that are in the headliner, engine air intakes, etc.
.....all of this still makes the use of a CO producing generator, unattended, at night, while sleeping even with detectors, very, very unwise. You just don't know where the wind, if any, or if it goes away completely will let the CO collect.
On the lakes here in TN and KY it only takes a very few minutes for the CO to kill, and that was only the CO that collects at the stern, under a shallow swim platform and such. Google for your own comfort