If you have a stainless prop you may find a high rate of anode
deterioration. The stainless prop/ aluminum case/ zinc anode
are your source of current flow in brackish or salt water.

Raise the motor.
That is the case and could well be the problem. Also, the motor is
mounted to an Aluminum jack plate which is bolted to the transom. I
wonder if this could increase the problem? Anyway, it looks like I
need to raise the motor when at dock which is not really a problem -
just a habit I need to develop.