Swallows and Amazons Forever!: The Big Six
Joe, Bill and Pete are members of the Coot Club which aims to protect wildlife, especially nesting birds. They spend most of their time on the river in their boat, the Death and Glory. Trouble starts when they're blamed for setting boats free from their moorings. Tom, Dick and Dot all become detectives and search for clues to lead them to the real culprit. They catch him just in the nick of time.
Starring
Watch "Swallows and Amazons Forever!: The Big Six" from streaming services Imdb TV (Prime), Imdb TV, Amazon released in 1984. It's a kids and drama movie, and has a rating of 7.9 out of 10 from Imdb which is usually fine for this kind of movie.