I agree with Jim. At least i3 owner with water sloshing around in the CFRP channel below the doors associated the problem with a botched windshield replacement that resulted in a leak into the CFRP passenger cell.
While drilling a drain hole allows the water to drain out, fixing the problem that allows the water to enter seems worth doing. Maybe you can learn whether the windshield has been replaced in your i3. If so, have your BMW dealer examine the windshield closely to determine whether leaking might be occurring around the seal between the glass and CFRP passenger cell. Replacing an i3 windshield can be difficult for those without prior experience.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE