Swallows and Amazons Forever!: The Big Six (1984)

Joe, Bill and Pete are members of the "Coot Club", spending most of their time on the river in their little boat, the "Death and Glory." Trouble starts when they are blamed for setting boats free from their moorings. Tom Dudgeon, the doctor's son, and Dick and Dot, children who are staying with Mrs. Barrable, all become detectives and search for clues to lead them to the real culprit.