National boundaries on the water are usually the 12-mile zone.
Maritime customs border
The seaward external border of the EU is also known as the maritime customs border. The EU border runs on the water along the national borders belonging to the EU. This is usually the 12-mile zone. Only where
the countries are too close to each other (e.g., at Fehmarn) the course of the national borders has been agreed upon. In some places, the countries are so close together that you cross the national border but not the EU. Knowing this border is important because when you enter the EU, certain obligations await you.
For example, there is a customs road obligation (in this case, you have to use waterways leading to a customs office) or a customs landing place obligation (you have to land where customs has an office place for clearance). If you don't want to do that, you must apply for an exception with costs.
And there is only in Germany the border of the coastal area (defined in Appendix 1 to the ZollV). It serves as the distance to be reached to obtain the authorization to
to obtain and consume duty and tax-free goods (e.g., alcohol, cigarettes) on board watercraft.
The border of the German coastal area is of great importance for the German Customs and all drivers of watercraft that obtain duty and tax-free goods. This line is still a relic from old times when the so-called "Butterschiffahrt" played an important role. This also explains, for example, the funnel in Mecklenburg Bay.