If the tap is exerting a force on the water, then its (the water's) momentum changes
The force is the rate of change of this momentum
Consider also Newton's 3rd
Play around with F=d(mv)/dt, note that the water is actually stationary to begin with, and that the water emerges with the speed given
In the end, you're looking to find that F=v*(dm/dt), where dm/dt is the given mass flow rate