As usual, we have at least two separate issues. Let's identify them, and maybe the question will answer itself.
The first is the iisu of clearing faults. This requires a low-impedance path back to the PoCo transformer, and the "ground rod" has no role. Often the neutral is this path.
Then there is the variation: a fault from a 'hot' wire to a metal case. That's where we need a good path between the fault and the PoCo neutral, and this is what we use the green wire for. IMO, "stray current" issues arise most often because this path is inadequate.
The "other" grounding has to do with lightning and static electricity. Those are the only forms of electricity that want to return to planet earth. Perhaps at some point boats with non-conductive hulls will be required to 'bond to water' much as we now require pools.
If a boat is not receiving power from shore, there is no reason for there to be any electrical bond with shore. In that instance, there needs to be a low impedance path back to wherever the electricity was created.
Now we enter the paradox of having multiple power sources. That is, should there be a failure of the PoCo neutral, then we have just energized the entire marina. Since fresh water isn't such a great conductor, we WILL have voltage gradients severe enough to kill. Your ONLY protection in this instance comes from GFI's.
