The Oker is a pleasant little river that is too small for commercial traffic but fine for canoes and other small boats. It flows right through Braunschweig, in fact it splits into two parts to go around the city center, which thus is on an island.
There are several places to rent canoes and other boats. The one called "Bootsstation Braunschweig" is centrally located by the bridge where the Kurt-Schumacher-Straße crosses the river, near Kennedyplatz.
The folks in the photo are paddling on the Oker River just south of Braunschweig, near Richmond Palace.
Yes, they do play American football in Germany. That's my ex-teamate from Simon Fraser University, Ian, on the cover of the program. The team in Braunschweig is called the Lions.