I dont know why this did not occurr to me sooner than this as I do a
lot of electroplating.

Put a thin layer of electrical conducting paint on the bottom of a
boat.

THEN: Run copper cables under the boat and apply DC so that you get
electroplating of the Cu onto the boat. Yes, it'd be slow in salt
water instead of a CuSO4 solution but it'd work. Every few months,
reverse polarity to clean the surface and then re-apply.