Groningen is located south of a sea located between where? 
Groningen (province):  It borders on Friesland to the west, Drenthe to the south, the German state of Niedersachsen (districts of Leer and Emsland) to the east, and the Wadden Sea to the north. Wadden Sea:  It lies between the coast of northwestern continental Europe and the range of Frisian Islands, forming a shallow body of water with tidal flats and wetlands.
Europe and the range of Frisian Islands