I did something like that many years ago before we went cruising. What I did was to stop by Home Depot and bought about 21 foot of copper tubing, you know the 3/8 or so stuff. I cut it into 3 foot sections. I used 2 to drop into the water on the sides and ends of the slip and measured the the SMALL voltage between them. There was some. SO, I then put them equally spaced, 3 to a side (with at least 2 foot into the water) on the each side of our slip, and one in the front of the slip. I then used very low cost wire (again from Home Depot) to connect all of them together. It stopped all action in our slip.
Just an idea that worked for us, your results may vary.