I successfully add a Mosfet to Arduino to control power and speed to 3 x 12v high-speed fans. It should also work just fine for your project even if you don't need to PWM the power to the 12v pumps.
Just ignore the potentiometer in the Arduino schematic, I didn't use that, just wire the MOSFET up like you see it on the breadboard and pull the Gate PIN high and you should get power flowing to your 12v motor.