When we were in a slip at a marina (MANY YEARS ago) I read some place how to handle this sort of stuff. NOW HEAR THIS! It might be snake oil, but it worked for me!
We had a houseboat move in just a few slips away. Before he moved in, my zincs were lasting around 6 months or so. Once he was in, my zincs were lasting a month or so. We saw he had a small hose washer & dry on his back deck. No chance of getting him to move, so I went researching.

I went down to Home Depot and bought about 6 foot of copper tubing, like the stuff for a refrigerator water line. And I bought (for my 30 ft slip) 75 feet of the lowest cost wire I could find.

Next, I cut the tube into 2 lengths. I secured one piece on one side of my port dock about center of the way out. I then secured the other across from the first piece. I connected the meter and read the voltage between the two at the most sensitive setting on the meter. Guess what? I saw about 1/4 volt (or so), as I remember.

I then went back to the store and got enough copper tubing to make 5 more lengths. I then secured them, 3 on one side, 3 on the other and one on the main dock at the front of the slip. I then used the cheep wire to connect all of them together. I made sure they were in the water to a depth of about my Keel.

After that my zincs were lasting 9+ months.

SORRY, wish I could remember where I read about this so I could give credit.

Greg