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.