#6 wire is not that expensive. I cant imagine you needing anymore than say fifty feet. particularly if you buy pool type clamps so you are not cutting into the wire and losing length. You want to do it that way also so you do not lose conductivity. Just get the anti corrosion compound. Generally it is used for aluminum conductors but will serve the same purpose on copper in a marine environment.
50' of #6 will run you roughly 55$
oxide inhibitor
pool lug
grounding wire splice block insulated and corrosion resistant. if you choose to go that route
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