Did they sell you a BMW warranty or a third party warranty?
Where is the water coming from?
If under the dash and coming from the center console, it is probably condensate from the A/C.
If from behind the trim in front of the door then most likely a clogged sunroof drain. If this, then, blow compressed air into the drain to see if you can free it. To test pour a small amount of water into the hole and see if the water is in or outside the car.
Regardless, find a new dealer.
Hopefully, yours isn't this bad.