The catamaran was travelling between the North Sea island of Helgoland and Borkum in the state of Lower Saxony. A wave several metres high apparently tore one of the railings off the boat and slammed it into a panoramic window. Glass shards sprayed onto passengers, three of which had to be transported to the hospital via helicopter. The other injured were treated in Borkum, police said.
Marine police began investigating the incident on Tuesday morning, a spokesperson said.