Some river users said the boat had remained around the same isolated inlet, between Wroxham Broad and Salhouse Broad, throughout the week.
Officers are treating the deaths as unexplained but are not looking for anybody else in connection with the inquiry. The girl is being cared for by family.
Detective Superintendent Jes Fry, of the joint Norfolk and Suffolk major investigation team, said: "We continue to investigate this tragic event.
"Our main consideration is the welfare of the 13-year-old girl and whilst I cannot speculate on the circumstances surrounding the deaths before I have received the results of the post-mortems, initial investigations confirm that we are not looking for anyone else in connection with the incident."
Specialist river policing officers found the 38ft-long Admiral hire craft on a stretch of the River Bure after being alerted by a boat rental company reporting that one of its vessels had not been returned on time.