Any electricians here please jump in if I'm wrong. I think what you are looking at in the above picture is the switched hot leg of the circuit going to the strobe/bell. There should be a green ground wire and a white neutral wire somewhere up the line. One side, let's say the blue is really on and if you measured the voltage between the white and blue wire you would get 110 volts. The orange wire let's say goes to the strobe/bell. If you measure the voltage between the white and orange wire without water flowing you should get zero volts AC. If water is flowing you would get a switch closure which would connect the blue to the orange wire. If you were to purchase a 110 Volt AC relay with a form C contact ouput you should be good to go. The relay will probably have three wires, green for ground, white for neutral, and either black, red, or some other color for the hot leg. If you hook white to white, green to green, and whatever the other color is on the relay to the dead i.e. the orange or blue wire in your picture that goes to the strobe/bell that should complete the wiring. Now, when the circuit is closed by water running through the pipes it will apply voltage to your strobe/bell and also your 110 volt relay. Use the form c contacts on the 110 volt relay to control a zone on the M1. Make any sense?