That would be a big "maybe" with at least as many interpretations as there are countries around the baltic sea. International law isn't as cut-and-dried as the usual laws within states that people are used to. Lots of international laws are more like traditions that got written down, where any major shift in circumstances (like a war, crisis, revolution, ...) will lead to a more convenient (for some) reinterpretation. Enforcement of international law usually happens through wars, so there is the additional complication of asking: "is it worth a war to uphold 'the law'". No real judicial and enforcement system exists, only certain kinds of arbitration courts. No international police to uphold anything. Only maybe the UN asking some states to send troops, if you really annoy the world and have no friends in the security council vetoing it.

The dam would have to be in German waters (German Bight, furthest point outwards is Heligoland) or far out in the North Sea (Edit: not in international waters, we're all close here, there aren't any between the continent and the UK). Denmark could in theory cut off the Baltic countries, but for international shipping, they'll often take the Kiel canal that cuts through Northern Germany between the Baltic Sea and the Elbe river (close to the mouth where it flows into the North Sea) because they're often delivering to Germany or transfer the load in Germany or the Netherlands to bigger ships that'll take it where ever it needs to go (and the canal route is considerably shorter).