Water where it's not supposed to be, cont'd
The oft-repeated first solution to try is to find someone who knows BMWs who can clear your car's drains. They get clogged with dirt, debris, bits of leaves and twigs, etc. and can't handle the water, so it leaks into the car. Bad drains in a Miata from years ago resulted in the same problem.
As someone stated in an earlier contribution to this thread, German convertibles don't leak, so yours shouldn't either.