Apparently they dumped stuff in the North Sea as well. The dumping locations that are known of are mostly the deeper parts. The Baltic is 459m at maximum depth while the North stacks a chilling 700m.
I think the dumping locations in the North Sea would be easier to find (The North has been mapped fairly comprehensively due to oil and gas exploration) but harder to deal with (it's tough as hell). The Baltic is smaller and surrounded mostly by land, so I'm guessing the Baltic would be hit harder.