Sub panel and ground rod installation: "high differential potential between the ground locations"

Question

We have a dock (fixed-pier) that is about 40 feet from the house, and +100 feet from the main panel. 4-wire (2 hots, 1 grounded neutral, and 1 ground) run to a large junction box at the start of the dock. I was thinking about replacing this with a sub-panel as I don't feel like this current implementation (done by someone prior) is to code (Generally speaking, when is it necessary to install a sub-panel for providing electricity to an outside shed/office?) and I feel like due to the distance it would be nice to have a disconnect at the dock.

My main question is sort of based on some conflicting information. I was under the impression that I would have to install 2 grounding rods 6ft apart near the dock and connect that with the ground coming from the feeder (main panel) and have the neutral from the feeder in the sub panel un-bonded from the ground. Thus, the sub panel would follow the diagram presented in the accepted answer here: How to properly ground a subpanel in detached building?

However, I was watching a YouTube video and read the following comment:

Greg Elkins: "There is one thing that bothers me, all grounds should
  be single point bonded, here have a separate ground run to the remote
  panel and yet installed a separate ground, this is bad. grounds should
  only be tied back to a single point bond to ground, have a separate
  ground run to the new panel, no additional ground rod should be
  located there. this allows for a dangerous situation that can have a
  high differential potential between the ground locations."

https://www.youtube.com/watch?v=oDor0hNmfL8&t=1554s

Is this user correct? Doesn't his comment go against: https://www.ecmweb.com/qampa/code-qa-grounding-and-bonding-remote-building

Retired Master Electrician · Answer

First, a marina dock is not a remote building.

Second, water and electricity is very dangerous. I learned this in my first year of apprenticeship training in fact it was almost my first lesson.

Marina, docks and boatyards are highly specialized. In fact the NEC dedicates and entire chapter on them, NEC Article 555.

Your questions indicate that you do not have the special skills to be wiring or repairing a dock. I am writing this because your personal safety and the safety of others is our first priority and I'm thinking you really do need someone to assist you on site.

Good luck and stay safe.

Sub panel and ground rod installation: "high differential potential between the ground locations"

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