The flapper cap won't work in this situation since the end of the pipe is vertical, cut off at an angle facing the rear where the sheared edge is vertical and the pipe is about 45 degrees to the aft. Perhaps there is a rain drain before it goes into the muffler, but I would hope it didn't put the rainwater down the deck due to the teak decking below the exhaust pipe.
There was a can / bucket over the end while it was at the dock so I assume there is a way to cover it up when not in use. Probably a telescoping boat hook or something like that.
I once did an overhaul on an engine that had a vertical exhaust and the bolt in the flapper cap vibrated out, leaving the exhaust pipe collecting water which entered two cylinders. We replaced the two cylinders and did a top end overhaul on the other two. That engine ran over 50,000 hours after that. I have no idea how long it had run before that point...