The permittivity, K, of water is about 80.

The relative velocity of propagation along a wire immersed in water is
about VF = 1/Sqrt( K ) = 0.11

At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water
is only 1.1 metres = 43 inches long.

Furthermore, in salt sea water, considering a wire as a transmission
line, dielectric loss is so high there is little or no current flowing
at the end of a quarterwave radial wire. Longer wires can be
disregarded because they carry no current.

So, at 7.5 MHz, there is no point in considering a system which has
more than a radius of 1.1 metres. At higher frequencies the radius is
even less.

A copper coin, 1" in diameter, immersed in a large volume of salt
water, has an impedance low enough to be used as an efficient ground
for a 1/4-wave HF vertical antenna. It is limited by its power
handling capacity.

I have made measurements years ago but have no records as I didn't
attach any importance to them at the time. And still don't.

Unpolluted, clean, fresh pond water, is a different kettle of fish.
Permittivity is still about 80 but the resistivity is very much
greater. About 1000 ohm-metres is a reasonable value.
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Reg.