Reliable, inexpensive way to be notified of a water leak?
Apart from notification, cut off the main water line when detecting a leak. For example WaterCop. It may connect to your existing automation system; see their FAQ:


  WaterCop systems are equipped with auxiliary input/output contacts and
  can be wired into your existing home automation system with simple low
  voltage wiring (wiring not included). Please consult your security
  system manual or installation professional for details on other
  systems.


I haven't used any such system.