Assessing your environmental impacts
If your water supply is metered then obtaining readings should be straightforward. 

Bottled water, whether in small bottles or as part of a dispensing machine, is extremely wasteful in terms of pollutants, energy in bottling, plastics and transportation.  Record any use of bottled water within your businesses separately from tap water.

Deciding on your targets
Although there are emissions associated with the extraction, storage and purification of water these are not usually calculated.  Instead, set a simple target of between 4.1 litres per person per day (best practice) and 12.3 litres per person per day (satisfactory). These figures are from the BRE Environmental Assessment Method.

Reducing your impacts
The first best step is to acknowledge that tap water in the UK is perfectly healthy to drink.  It is simple enough to store tap water in a jug or bottle in the fridge, and replace it as it is used up.  This will also reduce electricity demand compared to putting warm shop bought bottles in the fridge or operating a water cooler and dispensing machine.

Water use in toilets can often be reduced by putting purpose designed "water hippos" in the cistern.  Urinals that flush through the night and weekend should be modernised with proximity sensors or at least with time and day controls. 

Processes that have traditionally involved large consumptions of water are modernising.  Printing for example can now be carried out using waterless systems.  This not only saves millions of litres of water, it also drastically reduces many of the pollutants that traditional printing gives rise to.