A bilge pump is one of the most important safety components of a boat. The bilge pump drains water that accumulates in the boat. The addition of a float switch to a bilge pump circuit allows the bilge pump to come on automatically once the water in the boat reaches a certain level. This ensures that even if there is nobody on the boat to turn on the bilge pump, it will still pump water out of the boat.
Purchase a float switch that is compatible with your bilge pump. Most bilge pumps run on 12V DC power so it is important to get a switch intended for 12V power and rated for slightly more than the maximum wattage draw of your bilge pump. Purchase a float switch with long enough wires to run from your bilge pump to the pump control switch.
Disconnect the negative wire from your boat's negative battery terminal.
Position the float switch in the bilge near the bilge pump. The float switch should be at the same level or slightly higher than the bilge pump so that the switch does not trigger the pump before the water level has reached the pump.
Mount the float switch on the hull according to the instructions provided with your switch. Mounting methods vary from model to model but always use caution when working on the hull of your boat.
Run the wiring from the float switch to the current manual bilge pump control switch. Try to run the wiring in an area where it is out of the way, for example through the bilge, but avoid running it through an area where it will be continuously wet.
Replace the current bilge pump manual switch with a three-way switch. Connect the powered wire from the original switch to the line terminal on the three-way switch. Connect the wire that runs directly to the bilge pump to one of the switched terminals on the three-way switch. Connect one of the wires from the float switch to each of the switched terminals on the three-way switch.
Reconnect the negative cable to the battery's negative terminal. Flip the three-way switch into the "Off" position and listen for the bilge pump; it should remain off. Flip the switch into the automatic position and it should remain off. Flip the switch into the manual position and it should turn on. Flip the switch into the automatic position and pour some water into the bilge. Once the water level passes the float switch, the bilge pump should turn on automatically and drain the bilge. Leave the switch in the automatic position.