Wikipedia
The Achterwasser ( Low German achtern = "rear, behind") is a lagoon forming a bay on the Peenestrom river, which flows into the Baltic Sea, and like that estuary is an inland coastal waterbody. This lagoon extends so far into the island of Usedom, that it is only separated from the Baltic Sea by a narrow barrier spit between Ückeritz and Zinnowitz. It is bordered in the north by the peninsula of Gnitz and in the south by the Lieper Winkel. The reed-lined, south-eastern bay is called the Balmer See, the one in the southwest, which can only be accessed with difficulty on its western shore is the Krienker See.
In past centuries the barrier protecting the lagoon was sometimes breached by the Baltic Sea during storm surges that flooded across the island of Usedom, when the narrowest part of the land bridge between Zempin and Koserow at Rieckgraben (near Lüttenort) was flooded. This land bridge between the Achterwasser and the Baltic Sea is only 300 metres wide.
The Achterwasser is not very deep, and is a popular surfing and sailing area. The beach sand on the Achterwasser on the peninsula of Gnitz is not as fine-grained as that on the Baltic Sea beach of Usedom. The bottom of the Achterwasser is very stony.