Aboard Sanctuary, we use First Alert® smoke detector units that I bought at Lowes. I recommend the detectors that have *both* micro-particulate and ionization sensors. They detect combustion particles from microscopic incipient fire on up to visible smoke. They use a 9V “transistor radio” battery.
I located one in the vee berth, one on the master aft cabin, and one on the overhead (the ceiling) of the electrical closet. The single most likely place to have something start without occupants being aware of it is in that electrical closet. The alarm sounder on the First Alert is plenty loud enough to awaken a sleeping person.
First Alert and other manufacturers make detection systems in which if any one unit detects something amiss, it sends a wireless signal to cause all of them to alert. That would be good for boats under way, and for larger boats.
Most of the home units are rated for 100ºF. That rating is not sufficient for the boats engine room, although I think it would work. It’s also not enough for most garages in Florida. Further investigation required.
We have found that when using the oven for baking (lasagna, breads, etc; anything that takes more than about 1/2 hour), our vee berth unit will alarm. There’s never any visible smoke, so it must be the ionization sensor, but it does sometimes go off. We manage it when it happens. If we keep the boat opened up while baking (not too hot or cold for the boat to be open), it doesn’t happen.
We also have CO sensors. The CO sensors are made by Kidde®. These units are powered by 120VAC, but also have 9V battery backup in case we lose AC power. Since our utility outlets are powered by our inverter/charger, that rarely happens. We have located the CO detectors in our vee berth and in the master aft cabin, at the level or our bed pillows above the cabin sole. They are the type that shows CO ppm, numerically, in an LED window. They never show a non-zero reading, either from the main engine with the boat closed up, or with the genset running.