You only need to connect to one bronze thru-hull...the nearest one to the tuner. This thru-hull should NOT be otherwise connected to the boat's DC ground system.
won't matter much. After all, there's direct water contact INSIDE the thru-hull (unless you painted the inside, which is unlikely), and there's capacitative coupling as well.
Use a wide copper strip from the tuner to the thru-hull. Keep it as short as possible.
This is a workable, but minimalist ground system. It can probably be improved upon by running additional radials and/or tying into other large metal structures, like metal toerails, rubrails, etc. These do NOT have to be in contact with or in close proximity to the water.