Grounding of radio equipment.
The basic form of a RF-suitable antenna is the dipole antenna, a symmetrical shape consisting of two poles / legs, which would be attached to the masthead.
Since most ships are not two-masters, the installed antenna shape is not a dipole, so that the "missing" leg is compensated by means of a tuner and grounding.
Earth plates made of sintered bronze are the necessary link between the on-board electronics and the surrounding water as mass. The bronze structure has a very large surface to the surrounding water, since it is porous and water can penetrate. This design is much more efficient than a smooth earth surface.