Water usage is a little like Carbon, which is split into Scope 1, Scope 2 and Scope 3.
If you have metered water, then in the same way as Scope 1 Carbon footprints your water usage is easily discovered, similarly the direct usage at 3rd parties will be reasonably accessible.
Scope 3 - All other usage is where you get into embedded water. The water that was required to produce all the products that your organisation consumes.
Embedded water is the water that is used in the production of a goods or service.
It is important since water is a scarce commodity in most of the world and large amounts of water are often exported from dry countries as embedded/virtual water.
There is a good overview of embedded/virtual water at en.wikipedia.org/wiki/Virtual_water.