Driving a solenoid with a relay
All you would need is a 12V coil relay rated for sufficient contact current (for example, 10A relays are common and inexpensive). It might also be advisable to connect a diode (eg. 1N4004) across the relay coil (and another across the solenoid coil) to reduce sparking in the flow switch (and relay contacts). 

Panasonic JS-1A relays and similar are one choice for the relay.