Monopole radio antennas need a 'counterpoise', and for most purposes on land grounding the antenna system provides this counterpoise. The ground provides a reference voltage which, colloquially, serves to bounce the transmitted wave off your antenna. The grounding system ideally takes the form of a flat, horizontal 'radial ground plane', and is often a system of wires of different lengths radiating outwards from the transmitting antenna, and buried within the soil around the antenna. In soils of low moisture and conductivity, it has been found that pegging wires out horizontally close to the ground is as effective at creating a ground plane as burying them. The key factors are the lengths of the radiating wires, and their horizontal positioning, near the ground.
The ideal counterpoise is something with the same harmonic length as the radio wave being transmitted. The earth is an effective counterpoise, but it isn't the only solution. Just the easiest and cheapest if you are on land.
On boats there is of course no soil, but metal-hulled boats have excellent conductive connection to water, and water, especially salt-water, makes an excellent ground (but rather prone to conducting lightning strikes too, which might fry your radio).
What about wooden or glass-fibre boats - the vast majority?
The traditional approach has been to drill a hole through the hull, beneath but near the surface, to an externally-mounted grounding plate, and to run thin copper foil sheeting within the hull as a universal ground. This is of course prone to damage and corrosion, and needs frequent maintenance and replacement, and another through-hull that needs sealing and checking. (Nor does this solution protect your from voltage surges due to lightning).
Those days are over.
KISS-SSB is a thin enclosed bundle of copper cables of different lengths that can be laid out inside the boat, out of sight, and with no connection to the hull or to the water outside the hull. It is a purpose-designed counterpoise, with the different lengths of internal cable corresponding to the different wave bands used by marine HF SSB radios.
This heavy duty performer is designed primarily for fiberglass or wood boats using HF SSB marine bands. Input power up to 500 watts. Copper conductors are permanently sealed in durable PVC tubing.
It comprises a four foot lead that attaches to the grounding lug on your tuner and then you just stretch out the remaining 10 feet of 1' diameter tubing that encloses the array of specific lengths of radiating copper wires and coils that internally measure 600ft . It is just that simple, run it along the hull, behind a cushion, in the cabinets. If you can not run it straight it is fine to make sweeping turns, even a sweeping U-Turn is OK. The perfectly measured lengths of copper and marine sealed coils act as an exact mirror image of your radiating backstay, whip, or GAM antenna, and this is what bounces your radiated signal wave off your antenna.