So what is commonly done is a dynaplate is added to the boat. It's
basically a highly conductive copper plate that is through bolted to the
hull and is in contact with the sea water under the boat. There's a stud
on the dynaplate to which you attach a copper foil coming from the tuner
ground stud.
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Thank you for the above info. Now understanding the situation better , it
would perhaps be beneficial to have a 'tunable counterpoise' between the
transceiver ground connection and the dynaplate.
It is in fact an adjustable series tuned circuit comprising a air-spaced
capacitor and a roller coaster or switch-tapped type of variable inductor
which is tuned for minimum impedance with the aid of an integral RF current
meter ( as described by Roy ,W7EL) Tuning is done for each band by
maximising the current in the counterpoise at low power.
For the latter it is best to use a sensitive microamp meter ,its sensitivity
adjustable with a potmeter
Such a tunable counterpoise can also be used very well in a high rise
apartment where there is no good RF earth .
The end of the counterpoise is then not connected ,but since the RF voltage
can be considerable ,the free end should be well insulated.
Such a device can be home brewed ,but a version with a switch allowing for
several inductor taps is (or at least was) available from MFJ it's their
model MFJ-931 ; Artificial RF Ground.
It was (in the early 1990s ?) described in QST , by Doug DeMaw , W1FB
(sadly now SK)

I have home brewed the above type of artificial ground made with a roller
coaster inductor.
For the HF bands the inductor is to be adjustable between approx 0.9 to 43
microHenry ,with the variable capacitor being 200 -250 pF max.

Frank GM0CSZ / KN6WH