To be LEED certified (which is the international green building certification) you need to achieve 20% savings on water use, compared to the baseline of a usual building. 

Baseline water usage for regular used equipment indoors:

Shower                             9.5L / minutes

Toilet                                 6L / flush

Urinals                              3.8L/ flush 

Public Faucet                 1.9L/ min 

Kitchen Sink / basin    8.3L / min

Some ideas to reduce water usage by 20% in your spa:

• Catch rainwater and grey water (showers/washing hand sinks) in tanks and use to flush toilets and irrigation of landscapes
• Look out for Energy Star accredited hardware like taps, shower heads etc. that is more water efficient. Make sure they are 20% more efficient than the above listed base line. 
• Use energy efficient washing machines for towels and linen. 
• Use skin and body products that uses less water and steam during treatments and rather use hot compresses and dissolvable exfoliators that is just as effective. 
• Maintain your water outlets regularly. Leaking taps and broken faucets and valves can often cause water leakage that increases cost quickly. 
• Use sensors in public bathroom areas on switch taps on and off. This will prevent tap from releasing water when no one is there anymore. 
• Use dual flush toilets, so that less water can be used when only urination is performed. 
• Use air rated shower heads for a low flowing shower head but that still produces enough water for a comfortable shower. 
• Use only eco friendly products in showers and at hand basins so that the water can be captured and used in your garden. 
• Educate, educate, educate. Education of your staff and guest regarding the value of water is by far the most important part of keeping your water usage down. Eg. Place signage around your spa at water points, asking them to be more mindful of their water use.