Propane accidents are rare - especially when you consider how many boats are so-equipped. Carbon monoxide poisoning occurs frequently. There have been so many cases of carbn monoxide poisoning, including many deaths, that the Coast Guard in recent years has stepped-up it's campaign of warning boaters about the risk. Carbon monoxide can "pool" in unpredictable locations -- so fitting your boat with a CO detector(s) may or may not help you.
The big difference between the risks of propane and CO from gasoline combustion is that the propane appliance, when functioning properly, is not a risk. The gasoline appliance, when functioning properly, is emmiting a poisonous gas.