An apparatus provides a warning of propeller rotation or CO/CO2 hazard in a marine craft and includes a sensor in communication with the propeller to directly or indirectly sense rotation of the propeller and to generate a signal in response to the hazard, a controller coupled to the sensor to process the signal from the sensor, and a sensory alarm coupled to the controller to generate a warning alarm signal when the sensor returns a signal indicative of a predetermined state of hazard, namely actual rotation of the propeller from a nonzero rpm to a user or factory defined rpm or CO/CO2 levels at or above a predetermined maximum.