All you need is a good quality single pole single throw switch.

Let me explain:

All the manual/auto switch does is bypass the float switch. The circuit breaker is your on/off switch. A standard toggle switch can go into a standard circuit breaker panel. Wire the bilge pump for automatic operation. Use a three wire cable. The third wire goes from the manual/auto switch to the positive side of the bilge pump (same as the float switch). The other side of the switch goes to +12V. With the toggle switch open you are in automatic mode, with it closed the pump is running.

BTW, that third wire can also be used to light an indicator or sound an alarm. With my engine running or sailing fast I can't hear the bilge pump. I added a small pizeo alarm - easily obtainable at Radio Shack, so that whenever the bilge pump goes off I know it. Occasional alarms I ignore, if it is going off frequently I search for the reason.