Just my tuppence worth.....
Under the salt water, the RF (SSB) ground and the bonding to the zincs are electrically connected through the salt water.
I can't see it would make any difference which one you connected it to.
I can't see that in the event of a lightning strike or near strike, either will protect you.
None of our instrument "ground" connections are connected to anything.
The bonding is for corrosion protection for the metal parts in the boat that are in contact with sea water.
The RF (SSB) "ground" is the ground plane counter balance for the SSB antenna during SSB transmission.