More to the point, the system found on these is Direct Current, or DC, you are probably more familiar with Alternating Current, or AC, which is found on stationary object, buildings, homes, etc.
Without a good Earth, or ground, as described earlier, the electical system can not complete a circuit. This varies from land vehicular usage because the medium of travel is not conducive to proper circuits, ie: water. Cars, truck, motorcycles, etc, for the most part, have a matal framework that allows many portions of a land vehicles anatomy to be used as a ground, or earth. A wator-born craft needs a direct connection to the block for the ground to function correctly. The engine and other systems, all use this connection from the block to the battery as a ground, thereby completing the elctrical circuit.