The resistivity of sea water is high compared to metal, but very low
compared to air. But what counts is its intrinsic impedance compared to
that of free space. At 10 MHz, the magnitude of salt water's intrinsic
impedance is about 4 ohms, compared to the 377 ohms of free space. So it
does a good job of reflecting waves when air is at the other side of the
interface.

EZNEC will tell you the intrinsic impedance and other information about
any Real ground if you select Ground Info from the Utilities menu in the
main window.

Thanks Roy, very useful advice.

Tony I0JX