"Figment" wrote
My boat is never left without me first closing all sea cocks, which
includes
the 3" at the exhaust outlet.

Jeeze, sounds like a poor design. My exhaust outlet is a foot out of the
water then the hose is routed up under the gunwale to provide a trap. The
deck would have to be awash before water got past it and then where would it
go? Back thru the engine cooling system to a seacock that is below water
but kept closed unless the motor is running?