Here's a good paper on CO exposure: http://safetynet.smis.doi.gov/fae.pdf
In summary, boating through a 5 mph/no wake zone will expose people sitting at the back of the boat to 1,137 ppm CO (nearly 7 times the reccommeded levels). Wakesurfing, on the other hand, exposes the surfer to only 17 ppm CO, well within the safe limits.
MAYBE THEY SHOULD BAN 5 MPH/NO WAKE ZONES!!!:-)