Water in rear floor
I had the same problem, together with water leaking into the boot - result of blocked drains in the soft top compartment. I think that the water was filling up in the area of the blocked drains and then finding a way inside the car and ending up in the floor. Cleared the blocked drains and problem solved.
I managed to un-fix and raise the soft top mechanism on one side and cleared the drain (what genius at BMW decided to locate the drain there?!), then re-fixed and did the other side. I carefully marked the position of the roof mechanism so that it went back in the same place.