Here is a simple diagram for a automatic bilge pump. It uses a TIP 120 (Radio Shack Catalog #276-2068) Darlington Power Amp to switch the pump on when water reaches the two copper contacts. I use copper tape for the contacts and attach then to the pump housing (non-conductive!) near its base. For very small models the pump may be replaced with a flashing light or buzzer.
You may wish to add a capacitor across the motor leads to reduce RF interference. WARNING! Do not short between the two copper contacts. You will fry the TIP 120! Be careful when soldering, too much heat can damage the TIP 120.
This has been in use for approx 5 years with no failures.
Here is another version: