It may depend on the state, but when we sold our sailboat recently we where required to have a CO detector in the boat. That was diesel and 34' of boat. Some boats have the motors under or just aft of the main cabin and it could be possible to fill bilges up with CO. Add in bulkhead heaters and you have more risk. Propane is less of a risk for CO as a propane flame must have some real serious oxygen starvation issues to have CO in dangerous levels.
Our 21EC (2010) has no CO and since we did not buy through a broker or with survey we did not get one included.
CO detection is most needed if you are going to sleep on the boat with a running heater, cooker or engine.