On a boat, anything that can fail, will at the most inopportune time! This is why it is crucial to always have a backup.
Today, everything on a boat is switching over to the convenience of electronics. This is great until you consider the marine environment. The boat is a corrosion haven, constantly exposed to salts and humidity. Electrical components eventually will give way to corrosion and fail.
I'm not saying that we should negate all electronic conveniences from our boats to avoid the fatalities of corrosion. Instead I'm saying that it is important to remember to keep a backup, and preferably a manual backup on board for anything that you really need.
If we were holed and our electric bilge pump were to stop working we would have problems. We have two electric bilge pumps, one in the sump, the other mounted higher (the high water pump). The idea is that bilge scum and dog hair will likely foul the primary bilge pump, but the highwater pump will still be clean and ready to pump.
Such a wonderful theory, but the reality is that if we are taking on water, the batteries might run down and the pumps would stop working. Worse yet, if the batteries get submerged by the rising water, we will instantly lose our electric pumps.
This is why a manual bilge pump in a necessity. It will flow a large volume of water with each stroke. It does not depend on electrical connections or battery charge, but instead it works on you. They say "Nothing pumps faster than a scared man with a bucket". This is a little more sophisticated than a bucket, but as your fear builds, it pumps water out faster.
Can a manual bilge pump flow enough water? I found out when I was cleaning my bilge. My method for cleaning the bilge is to pour "Simple Green" into the bilge and let a hose run to fill it up. The soap will foam and the running water will eventually flush out all the bilge scum and dust that has accumulated over the past year. The manual bilge pump was able to keep up with the flow of a garden hose running full blast. I do feel confident that if we had a leak, we would be able to pump the boat dry (once the hole has been plugged).
If nothing else, having a manual bilge pump makes you feel like you are involved in your survival. If you stand around watching the water rise in the bilge, you might begin to panic. But if you are busy pumping your heart out, you will be kept occupied in your survival.