are you sure it's water. I would try putting a piece of tissue down to try and see if it's coloured. The only liquids I know of other than diesel and AddBlue in the rear of the car is screen wash. I wouldn't have thought it's either of those cos the filler is on the nearside of the car.
It won't be the aircon drain because that's at the front behind the engine by the bulkhead.
Traveller & 2 508s