The metal hull acts as a grounding

The metal hull acts as a grounding electrode to the shipboard’s
electrical system, both during navigation, when it is a TNsystem, and
when the vessel is berthed. The seawater, in fact, acts as the earth to
a land-based installation.2 Thus, an “earthing” conductor (EC) is employed
to link the shipboard’s main grounding bus to the hull, which,
therefore, becomes an EXCP (i.e., at zero potential). Supplementary
equipotential bonding connections may be additionally required between
hull and onboard equipment to reduce further potential differences
caused by faults.