If the engine compartment is closed and it is not moving faster than no wake speeds, the blower always needs to be on about 1 to 2 minutes or so before starting motor and keep it on until faster than no wake speeds or the engine is off.
Check the vent for fumes while waiting about 1 to 2 minutes to ventilate the engine compartment air before starting.
Doesn't matter if that is land on muffs or on the water, blower must be on at the times mentioned above.
I would say any time longer than 5 or at worst 10 minutes after the engine has been shut off warrants checking the engine compartment for gasoline fumes before starting.
What you have to worry about is fumes building up in the engine compartment.
When the engine is off or when there is not sufficient air flow to circulate any gas fumes in the bilge, the engine is not burning those fumes if present and there is no air flow to remove those explosive gas fumes.
