The pump is good and the switch sounds to be good from what you say.
I strongly suspect a wiring error - I can imagine both +ve and -ve both being wired to a single pole switch
Draw the circuit on the back of an envelope and see if you can make it work in your mind.
Supply - switch - load (pump) - back to the battery to complete the circuit.
You do not say where the zero volts is being measured.
Good luck...
July 2015
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