Shall I connect inverter with boat GND?
I think the "adviser" may have been talking about connecting the socket's earth to the inverter's neutral / cold / null point:







simulate this circuit &ndash; Schematic created using CircuitLab

With this arrangement, if there is a short or other fault causing current to flow through the earth connection, it will either cause enough current to flow to trip / blow the MCB or Fuse, or cause a mismatch in the live / neutral currents which would trip an RCD (GFCI).  You could also add an ELCB into the mix as well, and that would function normally.