There seems to be a lot of misinformation on the counterpoise subject. Here is an example of a patented commercially available system for use in any size boat: http://www.kiss-ssb.com/index.html
It appears to be a collection of different length wires in a plastic sheath. The reason for this is that a single length of wire will be resonant at a single frequency so in order to cover the large frequency range used in marine SSB they have used a collection of different length wires to cover the bandwidth required.
Many people have over the years just installed a copper strap to the bronze through hull fittings thinking that the HF signal needs a ground to the water but really all that is required is a length of wire to provide an opposing signal to the main transmitting antenna.
You could probably simulate the Kiss-SSB system by putting a number of different length wires onto the ground connection of the tuner.
Obviously the back stay needs to be insulated with ceramic insulators and I would place the antenna tuner as close as possible to where the actual antenna lead comes into the boat to minimise radiation inside the vessel.