What device can remotely close a water main?
So let's build your system.  For a remote water valve, try 



https://www.fortrezz.com/water-valves/

Once you have that working, you'll need a Z-Wave controller.  I've had good luck with the SmartThings hub:



http://www.smartthings.com

And then you can just pair them and it will show up as a device you can control via the Smart Things web app.

Unfortunately it will require some pipe work.  To do it in a way that pushes the current valve back and forth would require some kind of finicky hardware, in my opinion.  I also don't have any sort of HomeKit idea on it, but this seems to be a fairly basic Z-Wave appliance so it should be fairly easy.  It also runs on 12V, it does say you can hook it up to a car battery, but I'm assuming you could jury-rig some sort of 8-cell AA-system to get it up to 12 volts.