Possibly there is no filler under the rear seat, and the water from the pool falls under its feet. We need to check if all the plastic stubs are underneath. Another reason for this problem may be broken doors of doors.
There were also cases in which the water under the mat was dripping because of the leak beneath the windshield and the water ran under the skin of the front passenger, and from there it dripped back to the back. In this case, the windscreen should be repainted.
