That is an interesting question if you are working aboard a ship that is floating in water.

Usually when the discussion includes the word "earthed" it is referring to the frame of the welding machine being grounded to earth to ensure the circuit breaker will kick-out if there is a short circuit in the welding machine.

On a ship, things can become complicated by stray currents that can cause "increased corrosion" or corrosion like effects due to galvanic action introduced by stray currents whether they are from welding arcs or shorts in the equipment.

I have seen literature on this subject. I wish I could direct you to some reference source, but I cannot remember where I saw it.

Best regards - Al