[WCR1942]

Use a water meter to measure how many litres the HMs use in a week.
The following week offer them a basic food budget increased for every litre they can reduce the total by.
The advantage of this scheme is that it affects every aspect of their lives:
toilet flushing; cooking; drinking; showers and baths; topping up hot tub and pool.
Each of these areas will lead to endless arguments as different HMs waste water in different ways.

I have noticed over the years that many HMs declare an interest in environmental issues yet make no effort to save water. Watch some of them cleaning their teeth for example.