Just re-read your post;
If your motor is elec start , I'm betting on wiring problem .
I left a motor in salt water for the same time and only got marine growth , no corrosion .

Are you slipped anywhere close to some big powerboats , 1 or 2 slips away, thay could have a leaking electrical system.

I had a home made 'slip meter' to measure stray current in marinas. Basic VOM with special leads and 3' long probes . Works great in salt water.
Found an old Egg Harbour with enough stray current to shock a diver !