The definition "over" ground" generates confusion, because on the sea it is difficult to make references to the land, if not to the bottom that can not be understood as a 2D plane.
But it mainly means that it is not the speed of advance with respect to water, because it can be modified by sea currents.
To be clear, the GPS measures the SOG, while the helix-type contaminants, which are often present on the fishfinders, measure the speed in relation to the water.
Therefore, by SOG we mean the true speed free from the effects of sea currents.