I'm not a chemist, I'm an electrician.

What you have here is half a circuit.

Say you put the Anode at the front bumper of the car.
There IS an electrical connection from the rear bumper to the anode in the front bumper through the chassis.
But to complete the circuit you need to have current flow from the anode(front bumper) back to the rear bumper.
This is where the seawater and moist dirt comes in. It gets even harder when you realize you need to have the current run to every inch of the chassis.