The simplest waterproof switch I've seen was built by a friend in college for operating an underwater housing for his camera. Since he did not want any holes in the housing, which might leak, he used an inexpensive magnetic "reed" switch which was sealed inside the plexiglass housing with his camera.

A reed switch very simple and reliable. It is a sealed glass tube with two wires inside it. The wires are mounted close to one another in a vaccuum and a small magnetic field will cause them to flex and touch. Depending on the thickness of the wires used on the reed switches these devices can handle substantial current.

A magnet on the outside of the housing turned things on and off. He used a rectangular magnet, and made a track out of plastic strips which were glued onto the housing. The magnet could slide to either end of the track, but could not come out of the track and be lost. It worked fine through 3/8 inch thick plexiglass.

Reed switches are available at Radio Shack on occasions, or at electronics stores. A more expensive version is sold as a window or door switch for a burglar alarm system. There are also reed relays which are simply reed switches stuck into a coil of wire. (You can remove the reed switch rather easily and use it by itself.)

A reed switch can be glued to the side of your pump, or completely encapsulated in some epoxy resin. If you stick the magnet onto a float, as it rises it will pass by the embedded reed switch and turn on your pump. When the water level in the bilge drops, the magnet goes down, and the pump shuts off. Since the magnets are not very heavy, the float can be rather small.

A similar device, is a "hall effect" switch, which is a solid state device that detects magnetic fields. There are more sensitive (you can use a smaller magnet) and also more expensive. They may not be able to handle the amperage of your pump directly, in which case you wire them to a small relay, and wire the relay to the pump. hope this helps.

Paul G. Jacobson