Knowing your bilge pump is running often, or perhaps continuously, can give warning of a potentially serious problem.
The operation of the pump is likely to go unnoticed when the boat is underway. A simple panel light that illuminates every time the pump runs can inform you that your bilge is not staying dry. You can buy bilge pump alarms that not only show a light but make an alarm sound when the float switch or another device activates them.
Twelve-volt LED panel lights are small, bright, and cheap. Mounting is a matter of drilling a hole in the instrument panel or other location and securing with a threaded nut from behind. LEDs are polarity sensitive, so be sure you connect the negative lead to ground. It should be marked with a minus sign but could also be indicated by black insulation or a lead that is shorter than the other one.
The power to illuminate the indicator light is provided by connecting the positive lead — marked with a plus sign, red insulation, or the longer wire of the two — to the pump side of the float switch. If the float switch is a separate unit, simply splice into the wire from the switch to the pump. The best way to do this is to cut the float switch wire, then rejoin it with a step-down butt-splice connector, inserting the indicator light lead into the larger side of the connector. Use a heat-shrink butt connector or buy heat-shrink tape and apply. Be careful of the heat source. Cover the heat-shrink with a good coating of silicone. Make this connection well above the water level and secure it in place so that it won’t slip into the water.
If your bilge pump has an integral float switch, it should also have a third wire for running the pump manually. Whether you have this wire connected to a manual operation switch or not, because it attaches to the positive side of the pump motor internally, it will be energized even when the pump is activated by the float switch. Test, however, to be sure you pump is wired in this manner and that the alarm will work for this type of pump. So you can connect your positive panel light lead to this wire.
Note that if you have the kind of bilge pump that checks for water by cycling and sensing load, your indicator light is going to flash every 150 seconds or so. That doesn’t make the warning light any less useful, but you might consider a lower-intensity bulb to make it less intrusive.