Maybe one of those "grouper" or guppy" zincs could help with the corrosion issues. Perhaps a homemade model.
Those old Yamaha 2 stroke outboard lower units are very good about surviving in salt water. I've been casually looking for a backup to my 1992 8 horse. A used 6 hp Yamaha longshaft outboard is very difficult to find. The 8 hp is even more rare. It seems like a good one sells for about $500-$700