"Ground?" Sure, makes sense - as long as you don't think about it!
Except when connected to shore power
, "ground" is a misused term on boats. Seawater isn't copper: it's conductivity is very low. Stick ohmmeter probes in it and see for yourself. It's better to think of a boat like an aircraft in flight. Want a "ground"? OK.
- fill a bucket with dirt,
- stick a copper rod in it,
- and call it "ground."
It'll be completely useless, but if it makes you feel better...
In AC power
distribution to homes, one side of the mains supply is connected to an actual earth potential (neutral) just outside the home for lightning protection. In the early days when AC power floated above ground, people objected to having lightning flying around in their homes (side strikes).
The correct term is "common," short for "common potential." But people long ago confused "common" as a ground - first in automobiles (which are, because of their rubber tires, insulated from earth potential) and the term got stuck with people who didn't have an electrical background.
Connecting all conductive objects to "common" (including, by convention, the battery
negative terminal) prevents, or at least reduces, a destructive "side strike" if the vessel is struck by lightning. Side strikes (also known as "sideflashes") start fires.
A description of the rationale is described in National Fire Protection Association standard NFPA 780: https://www.nfpa.org/codes-and-stand...etail?code=780
. It has a section specific to vessels. You have to pay for that document; here's a similar free document: https://lightning.org/lightning-protection-overview/
Preventing galvanic corrosion
is a separate, and much more complex, topic.