Why would we treat this any different than a swimming pool or a agricultural building. If you create an equipotential ground reference at the dock with the land or the water I think the problem goes away.
If you just want a quick experiment. put a ground rod clamp on a 10' stick of EMT or rigid and drop it in the water, connect that to the EGC. Test it.
If that makes the trouble go away you should look for a more compliant grounding electrode near the dock.
It is really not that unusual to see a couple volts ground shift between systems 100' apart. The worst I ever saw was more like 35 volts.
