Hey Dave,
...and simply removing the cowl vent and replacing with a deck plate is no guarantee, as green water sweeping the deck can still get in through the drain holes at the base of the dorade box...
My pipe coming through the coachroof is made of 3" PVC, and I simply put a PVC end cap on it if I'm expecting conditions where the dorades might become momentarily submerged...
best regards,
Jon