
Switching Channels (1988)
I was living in England by the time this film was released on an unsuspecting public. But, having a weather eye open for familiar settings, I noticed that the weekend away scenes were shot with a backdrop that looked very suspicious, to me: Windermere, on Lake Rosseau in Muskoka.
Here’s a still photo from the balcony scene:

Those Muskoka chairs are a huge clue. Even the boathouse in the background on the dock piqued my curiosity. Another shot of them on the water has a boathouse in the background that was always a landmark during my childhood years (see the film review clip, at 5:05 above).
Here’s a snapshot:

Once seen, never forgotten.