gepe wrote:I had actually hoped that someone knows the internal setup and where to drill a hole so the water can escape. The water probably is trapped in there because of an imperfect weld seam or something like this because only one of the two chambers has the water trapped inside.

Andy has explained about the perforated inner section on one chamber. This perforated section has the second, outer 'skin' welded over it.
That would make sense of why you only have water trapped in one chamber.
If you are drilling, or going to drill into the section with water trapped in it, you will have to get the hole welded up again.
If you don't weld up the hole, you will have a leak there.