I have the same engine box shown in your picture. With the water diversion channel, I would only get a few drops of water on the engine after a heavy rain or after washing the boat. I did notice that the channel was diverting water to fall into the bilge so I removed the trim pieces on both sides of the channel by drilling out the pop-rivets. I then installed 2" extension pieces similar to the original channel. This did eliminate rainwater falling into the bilge but I would still get a couple of drops on the engine after a heavy rain, probably splatter this missed the channel.
I run the boat out of Jones Inlet where 5' breaking waves are common. I have had waves break on the pilothouse and over the top into the cockpit. My concern is if even a few drops of saltwater make it through the 1/8" gap between the top of the box and the transom, the turbo will suck the water in. Diesels suck a tremendous amount of air.
I am looking for a rubber gasket with a 1/4" grove that I can attach around the back edge of the engine box.
Thanks,
BlueSky