Location: New Canaan, CT
Home Port: Brewer Essex Island Marina, Essex, CT
Where are you exhausting it? If through the transom you should certainly consider a flapper. It prevents water from being pushed back through the exhaust and flooding the generator. Following seas, backing down hard, or a stern on sea from a storm can all be a threat.
That said, as Jeremy notes, your muffler may give you protection.
I have a small flapper on mine. Cheap insurance.
2006 35TE Convertible, Volvo D6-370's
Former owner - Sonny III, 1997 28TE with "The BEAST"