dave, be afraid, be VERY afraid.

300 people die each year by falling off a boat and drowning. If one person a
decade dies of CO poisoning on a boat, it is a whole lot less of a worry.

what are the odds? are they greater or lesser than dying in a rear-ended car
accident waiting for a stop light on the way to the marina?

You can do something about the risk from CO poisoning, while the stop light is
just pure dumb luck.

Of course, if you want to allow your exhaust system to decay and fill your boat
with fumes and then say, "hey, no big deal, I could have been killed just as
easily driving to the marina", that's a personal choice.