Just my opinion and this is more of a trailer sailor thing where you pretty much always have an ungrounded mast. . A length of 4 gauge wire makes a much better conductor compared to chain and is probably lighter to carry on the boat. Wrap the wire around the mast to form a spark gap, drape the wire so that it goes near the side stays (to also form a spark gap). The ends of the wire only need to get near the water surface. Some sort of underwater ground plate only works in salt water anyhow but the end of the wire near the water surface has some chance of re-sparking and dissipating the charge on the water surface in fresh or salt water. Even if the wire were to extend below the water surface, it would like still re-spark and jump to the water surface.
The 12 volt transorb voltage clamps have to go on every wire that comes into an electronic box you are trying to protect. The OP found some device that works on the communication lines and these will be the most sensitive. But every wire can pick up the magnetic field from the lightning strike - even if the lighting strike did not hit your boat.
This is all about dealing with a strike. Trying to avoid a strike.. good luck might be your best bet.