Magnetic Bilge Pump switch

The large upright tube has a smaller sealed tube loose inside of it. The smaller tube has a couple of SS bolts in it to give it weight and a magnet inside the top of the tube. The larger tube has a SS switch made from SS with a magnet on top of it to pull it down against the contacts. 

I chose SS because of corrosion resistance.

When the smaller tube floats up the two magnets attract pulling the switch closed and turning on the pump. When the water drops the added weight of the bolts in the smaller tube pulls the two magnets apart allowing the springiness of the stainless steel to open the contacts and stopping the pump.

The magnetic switch actually operates a 30 amp relay (which operates the pump) so the amperage on the SS contacts is not enough to burn them.