What is it and how do you know you are succumbing to it?
Carbon monoxide (CO) kills you by attaching itself to your blood cells in a way that keeps your blood from picking up fresh oxygen as it passes through your lungs. You essentially suffocate slowly while surrounded by air. Little by little, the CO hitches a ride on your blood cells until there are no seats left on the life-sustaining bus (your blood) for the oxygen to sit. As it happens, you start to get headaches, muscle aches, excessive fatigue and nausea. As I tell my seamanship students, “If you are starting to feel like you are coming down with the flu, but you don’t have the sniffles, get out of there! You may be dying of CO poisoning!”
What do you do now!
Say you enter your cruiser, and you find a crew man lying unconscious on the sole of the cabin. The manual actually says that you are first to try to isolate the source of the fumes, extinguish them and proceed to get your mate to fresh air (to start the cleansing cycle of kicking the CO’s out of their seats and replacing them with O’s!)
Here is where I am coming from. This strategy could cause two people to die from CO poisoning, not one. IF you know that the only source of CO poisoning is YOUR engine, and you can kill it in a virtual flash, do it – or get out of there immediately and run for help! If you have to play Sherlock Holmes looking for clues, you are playing with your life. Remember, it could be your slip-mate’s engine that is polluting your air. Also, the cabin has built up some degree of CO concentration. It may be a deadly amount and you don’t have a chemistry kit in your pocket. You are gambling with your life.
You certainly can  grab a lungful of fresh air from outside, hold your breath and  try to drag the person out of the cabin. If you can’t,  drop them and run for help. If you can get them out, but they are not breathing, give them artificial respiration (sometimes called CPR) and scream for help between breaths!
How to prevent it?
One, create fresh air paths that keep the station wagon effect from overtaking you from behind. Don’t form that cup – crack a window somewhere and create a jet-stream through your boat. Just be aware that running on a “slow bell*” may still create an opportunity for CO to build up in your cockpit. Two, put carbon monoxide detectors in ALL enclosed spaces, especially sleeping quarters. These devices can be battery-operated and are readily available. $29.95 at places like Home Depot, Wal-Mart, the internet, etc… There are models that can be tied directly into your battery system like a bilge pump – never fails unless the battery itself is dead.
I’ve heard these devices. They can wake the dead. Almost. Be aware. Be safe. Always prepared – semper paratus…!
* slow bell is putting just enough way on to maintain bare steerage. Back drafting can occur under such conditions – and you’ll never smell it.