The connections illustrated above are typical of all wiring to a ship-board audio system.
Some ship owners opt to put a switch (DPST) at the control console. This allows the skipper to switch from LOW to HIGH GAIN modes. In this case the switch will connect the YELLOW wire (switch common) to the BLACK or RED hydrophone wire.
For safety the RED and YELLOW wires from the Hydrophone have resettable fuses inside the block that these wires come from. In the case that the Hydrophone Cable is cut or damaged, these fuses will open, but when the damage is repaired, the fuses will automatically reset.