Ok i have had the same problem as you.. I had water in the boot and also where the rear seats were.

- First of all find the leak by taking of all the trims, seats, boot carpet etc - it might take some time to find the leak but its worth finding
- I found that the leak was coming trough the rear lights i dont know if this will be the same for you but its worth a try.. When you take of the rear trim that reveals the rear lights see if you can spot any leaks or water coming from where the bulbs are inserted into the light.. Usually the seal between the light and body of the car is worn out and doesnt stop the water from coming into the light and then the water drips down into the boot through the screw holes and stuff..
- Then when u accelerate/brake the water shifts up and down the boot, then coming into the place where the rear seats are..