I spoke with the engineer last week. He told me they found the 600-amp panel on the dock was single-phase. The service is 3-phase. Parallel (3 sets) W cables were pulled from the service disconnect to the MDP on the dock. The cables were 4-wire with 2-hots and ground being used leaving one unused conductor.
They grounded one end of those unused conductors...
I fail to see any purpose in grounding (bonding) one end.
...and reworked the grounding on the transformers on the dock...
Without the how, that bit of info is almost useless.
...and got the stray voltage lower. ...
This is the only info that keeps the "reworked" info from being totally useless. Doubt bonding one end of the extra condutors contributed to this. In reworking the grounding, they apparently decreased the resistance from the dock to ground and water.
...They did determine it is some kind of capacitance and the engineer thought the plastic filter fabric might be acting like an insulator in a capacitor (but he thought it was a stretch).
I'd determine the level of system leakage before persuing the capacitive improbability. Even if there is a capacitve effect between the dock and water, coupled with varied resistance between dock and grounded metal parts, the dock has to be getting it energy from somewhere other than the water and grounded parts. That leaves system leakage or inductive coupling between dock and energized conduuctors, or a combination thereof.
The voltage (deck screws to water) gets lower the closer you get to the transformers. The EE thinks the transformers being bolted to the dock might be pulling the voltage down since the transformers are bonded by the EGC.
That's a reasonable deduction being the dock is not a low resistance path.
The EE said he didn't think it was a big problem and a real head scratcher.
Until the voltage source is determined it is a big problem. Until such is known and remedial action taken, the possibility of the problem getting worse rather than better also exists.
I suggested he have the electrician lift the line conductors at the transformer and see if the voltage goes away. If the voltage is still there with the line side of the transformer disconnected then you can concentrate on the feeders ahead of the transformer. The EE seemed to like that idea and he was going to try it. I haven't spoken with him since then.
Need to determine and follow a controlled shutdown with documented testing at each stage.
I'd start by opening each secondary disconnect working from the farthest back, and measuring local voltage from dock (screws) to ground and water before and after each disconnect opening.
If that don't expose the problem, do the same process while deenergizing the primary feeders one tranny at a time. If the problem didn't go away, you have stray current from another system. If the problem goes away, disconnect the feeders from each tranny and megger the trannies and feeders.
Having only 2 of 3 phases down there makes me feel even stronger about my theory of the out of balance feeders developing a small field that is being induced into wet, treated with copper wood.
With what info has been provided so far, I certainly cannot rule it out...