Can you mount a hair dryer near the starter to dry it out after each run

Forget about fixing the cause of the problem!
Id get a seamstress to make you a custom starter cover out of some old wet suits, baller!!!

Maybe install a grease zerk on the starter housing , and pump it full of marine waterproof grease

The is failing in the Bendix or electrically? Just to eliminate another possible leak area, check to see if you have the blue plastic wing nut block drains. If you do you could be missing the o-ring or it could be cross threaded and will only leak well the engine is running and underway. I had a customer (new to me) with a complaint about a fuel issue. He had multiple fuel filters on board and was told to just keep changing them. Upon my initial inspection I found a tremendous amount of rust on the underside of the exhaust on the starboard side (Merc EFI, flame arrestor starboard side). During the test run and at about 1800 RPM's and up it was spraying water all over that side and just enough so the engine could ingest it. So just check to make sure it is not that simple.