You are unlikely to have CO present without the odor of engine exhaust present. The lone exception to this is in the case of leaking engine exhaust systems. Just be aware that over a period of time people become inured to the smell and cease noticing it. Remember that you are unlikely to hear a CO alarm over the engine noise. CO gets into the boat by two means.
Some boats are more prone to the station wagon effect.
Typically these are flying bridge sedans with a high aspect ratio that will tend to pull in fumes behind it. Boats that are least prone to it are express cruisers and other similar low profile boats. Very calm days are more dangerous than windy days. Heading into the wind, when there is a great deal of air, spray and exhaust fumes being pulled aboard, you may get frequent whiffs of exhaust fumes, but chances are there is so much fresh air moving around in the cockpit that it's not going to be harmful. The larger danger is when the air is calm, and when the fumes being pulled back stay there and are not blowing away.