"Listed to UL 2034 rigid marine standards."
I suspect that's what you are paying for. Submitting something to UL can cost the maker $50,000 and that raises the unit costs. But this UL standard is easily found online www.protechsafety.com/standard/ul2034.pdf
and apparently there is nothing "marine" about it, it covers home and boats alike. So I'd expect that ANY UL-listed CO detector from the hardware store would be just as good for use on a boat.
WRT the cheap smoke detectors--I've had the pleasure of seeinf firemen outside my door and smoke on my ceilings while all the damned smoke detectors DID NOT SOUND OFF. Apparently the cheap ionization types are relatively insensitive, and if your goal is safety, not just meeting rules, then you really want a 'dual' mode type that is photosensitive and ionization detectors built in one. About $30+, but much better protection.
What that says about cheap monoxide detectors....well, I live someplace where code requires one. And I believe in them. But since I always keep a window open, and there are no
combustion sources, no
gas burners, no
adjacent flue vents, chimneys or gensets...I pulled the batteries out of that because I will never need it, and don't want to be awakened in the night when they need changing.
Next they'll require me to buy flood insurance....despite the fact that I'm on top of a hill and far enough above sea level that even a tsunami couldn't flood me out.