Peter,
as I recall, didn't W1BB go through puzzlement like yours  during his
extensive top band activities about forty to fifty years ago (that's my
generation gap showing again) operating from the middle of a salt marsh?
Bound to be something in the ARRL archives about it tho' if I remember
aright SWM also carried reports.
At least you have a respectable amount of antenna current. My modest 1.5A
into 3 wire 27ft vertical with about 630ft of  criss-cross top loading seems
to vary with the number of starlings and goldfinches using it as a meeting
place.
As I am on boulder clay on top of limestone, even getting earth rods to go
in, let alone make measurements, is difficult enough. However when building
on the 'west wing' last year I got the builder (GI0VLE) to put down a 5 wire
fan of heavy earth cable under the founds and have that plus the four 4ft
spikes of the old station earth tied to the ( all copper)water and heating
pipework in the bungalow, which is in turn connected via copper to the iron
watermain in the street, and for good measure to redundant power and
telephone earths from who knows where..  As a station earth it seems to make
little odds on HF but it don't half help the LF.

73,

Ian