A boat. A lake. A jetty. An island.
The Steinhuder Meer is the biggest lake in the Lower Saxony region, around 30 km² in area. It isn't very deep, according to Wikipedia its mean depth is only 1.35 metres.
The island in the photo - Insel Wilhelmstein - is artificial. It was constructed between 1761 and 1765, and a fort was built on the island between 1765 and 1767.
There's a 35 km bike trail around the lake, its all quite flat and not difficult to negotiate. This view is from the southern shore, to the west of Steinhuder.