I had the same prompt pop up after 2 hours on the motor, took it to the dealer and it had a faulty 02 sensor. Replaced it and it has been good ever since. If the motor is running with no problems chances are it is an 02 sensor, you should still be covered under warranty for this.
If you cycle the boat on/off in a short period of time sometimes this will also trigger the prompt and eventually will go away. Also try shutting down all batteries and your master breaker switch.