In what context was he using this statement as it relates to the DC system on a boat???
Theoretecally and ideally, earth ground is an infinite source or sink of charge which means it can give electrons to a positively charged object until it becomes neutral and vice-versa.
On a boat, you might have a floating ground, a ground connected to earth ground via a seawater connection or you may not even have a ground at all. Either way, I would not consider your boat ground as an infinite source/sink of charge and certainly not something that is going to magically provide a source of power