MY AC comments are entirely secondary. But if asked about DC, I would say that best electrical
practice is to isolate DC from the hull as far as practical, this should include isolated alternators, starters and senders.
Oh course in practice, while this is commonplace on good aluminium vessels, its less common on steel. However that does not negate the principle.
Connecting the hull to DC negative, facilities subsequent impressed corrosion through wet bilges and other inadvertent connections.
I fully agree that the differential voltage across the hull plating are not generally an issue, though resistance at welds can be an issue in high currents, though not applicable here
As to ABYC, I would argue it is not applicable to steel vessels in the main
Michael Kasten also agrees saying
"Keep in mind though that the ABYC rules represent the consensus of the US Marine Manufacturers Association, and are therefore primarily aimed at satisfying the requirements aboard GRP vessels,"