In addition to Pondlife's answer, I am contributing some more details.
Airbus 380 has room (see page 250) for potable water tanks which can be added to existing on-board water capacity for showers etc.
As we know, the showers aren't available for everyone but only for first class, which is probably less than 5% of the total number of passengers, so the water usage is not tremendously huge.
Waster consumption and shower duration is also controlled. Franke makes the showers for Emirates and states:
Low water consumption of less than 5 litres per minute
When 75% of the allowed shower duration is consumed, it shuts down and you have to turn it on again for the remaining 75 seconds.
Here is a first hand account (including video) of a passenger enjoying the spa.