An autopsy determined the woman was killed by strangulation before her body was thrown into the water in the Norfolk Broads, The Daily Telegraph reported. Her body and the body of her suspected killer were found Sunday after police discovered the 13-year-old on a rented cabin cruiser.
The man drowned, police said.
"We are not looking for anyone else in connection with this incident," Detective Inspector Jes Fry of the Norfolk and Suffolk Major Investigation Team said.
Fry urged the news media to leave the girl in peace, saying she is now being cared for by relatives. She was apparently too scared or traumatized to seek help until police went to the boat.
The Norfolk Broads is a network of large shallow openings in rivers and a popular boating area.
Local watermen said the boat had been tied up in one spot in Salhouse Little Broad for days. A water-borne ice cream vendor told the Telegraph he approached the boat last Tuesday because he was concerned, only to be waved off by the man.