You're right that a reed switch will not handle the current required, even for a small 12V pump. If you reduce the voltage/current to the pump to a level that the reed can handle, the pump won't work. That's why you will need an intermediate relay. The attached schematic is a switching circuit that should do what you want, but that relay is rated at only 2 amps. If your pumps draws more current you would need a different relay and other component values.
Ken