[Deadeye]

Marine applications are one of the few instances where "negative" and "ground" are not synonymous.

"Negative" deals with the current carried from a battery, genny, etc whereas "ground" refers to the current generated by dissimilar metals within an electrolyte (seawater).
Right away, you can see that the grounding system's purpose is to prevent the boat from becoming a battery by making sure the "battery's" electrodes (underwater metal components) are shorted out and allowed to dissipate through the sacrificial anodes (zincs)

The ABYC bit you quoted specifically forbids using any of this metal as part of the grounding(bonding) system. Essentially, you are electrically connecting all the underwater metal components. In your example, if the battery negative were disconnected, then obviously the engine wouldn't start. Ignoring that for the moment, the zincs would begin to corrode much faster. After being eaten away, more expensive metal begins to disappear.

In building aluminum power boats, we would bond all ac components and all significant hunks of metal together with 4 ga wire (green). Otherwise, the practical results are gremlins like hot water tank and heat exchanger zincs that don't last the season.

A marina shorepower hookup can make this even worse because now you can have two adjacent boats acting as individual electrodes.

This is all completely independent of the DC power system.