Carbon Monoxide Detector have gotten so cheap, get one and if you don't like it take it home and use it there. I would get one, with the read out so you can see what is happening.
I am guessing it will be fine on a boat.
Mounting location: I would put in the main saloon A little away from engine and stove. CO2 is heaver then air but is so close to the weight of air it can pool high or low.
The sensor are funny, a over load of chemical can set them off ( like oiling all the wood work or painting below ) it can take a sensor over 24 hours to recover to a base line. So if you are having trouble with one set it out in clean air for a day then try it again.
This information is at least 10 years old but I still think it is right.