Sounds like a 'hot slip' or a wiring problem on the boat.
1- Get a heavy cable and attach a bunch of zincs to one end , the other to your ground . Hang in the water at the slip. Or go to West Marine and buy a 'Guppy Zinc' .
2- Pull the motor out the well , thats what I do . $100 buys a lot of BenGay ointment.
4- Is that the right prop , pitch wise ? Cavitation can cause your prop to erode too , and too much pitch will cause cavitation .