Reg, G4FGQ wrote:
"At radio fgrequencies the impedance of the connection to ground will be
that of the connecting wire only, even before the resistance of the
connecting wire to the water is taken into account."

In general, Reg is correct.

I`ve installed a marine radio on a yacht, engine powered not the sailing
kind. It worked well as expected and the owner paid promptly. I`ve also
installed radios of various types in many vessels including large ones
used on the high seas and small work boats serving the near offshore.
But, I never installed an antenna using radials on a boat or ship.
Radials can be hazardous to eyes and body parts at sea. For VHF, a
1/2-wave coaxial antenna has the same gain as a ground plane but needs
no radials. For HF, it`s easy and effective to use a loaded vertical
antenna against the sea as a return path.

Best regards, Richard Harrison, K5WZI