Swallows and Amazons, the first book of the eponymous series of children's classics, as well as many of the later books in the series, is set in and around a lake in the Lake District of England. I don't remember whether the lake is named in the book, but it's certainly fictional, albeit based on real-life locations that Ransome knew. From Wikipedia:
the lake is a fictionalised version of Windermere but the surrounding countryside more closely resembles that around Coniston. Wild Cat Island, the location of the island camp, has elements from Peel Island in Coniston and Blake Holme (or Blakeholme) in Windermere.
However, the references are to other books, extratextual material, which doesn't seem to be available online. I'm curious about how clear these connections are from the text itself, including the maps at the front of the book. Is it obvious to anyone who knows the Lake District which lakes and islands the fictional setting is modelled on?