Carbon Monoxide from the exhaust can be pulled along by the boat in the turbulence behind the transom. They call this the 'stationwagon affect'. CO takes more than a week to completely leave your system so there is a cumulative effect on your blood stream. If you are exposed you should avoid being exposed again for at least a week. It is a dangerous thing.
If you think you are suffering the effects of exposure, and your symptoms are typical, then move to the front of the boat, ahead of the windscreen, and make sure it's turned off when you are loading or unloading riders. Be careful on the drive home.