How much water does the Emirates A380 carry in order to provide onboard showers?
570 litres, apparently:


  To cater for the showers, the airline's 489-seat A380s are fitted with
  two optional auxiliary potable water tanks installed in the centre
  wingbox, which increases capacity from the standard 1,700 litres
  (450USgal) capacity in six tanks to 2,270 litres - ie around 2.3t.


That's assuming that the 570 litre increase is entirely allocated to the showers, which may not be literally true.