I had a similar problem in 2016 and it was traced back to a fault on the aircon.
The pipe that takes away condensation passes through the car in the passenger side front footwell.
If that has come loose (poorly connected in the factory) then the water can run down the outside of the pipe and then when it hits the point where it exits under the car the seal at that point prevents it from dropping throught the floor, it builds up under the carpet and will naturally flow back into the rear footwell when you accelerate. In my case it came up through the connections for the mats in the rear and I had standing water in the rear footwell. Seems like I have the same problem back on the replacement XF I got when I traded that particular factory lemon back in. 4 years later I now have standing water again, but this time in the rear driver side footwell.
The lesson to me now is never ever buy a new Jaguar, the quality control is dismal.
