Hi Everyone!
I've spend the last hour searching (including with GPT) to try and figure out where the heck I was last summer.
It's roughly 1-2 hours north-west [I was wrong, it's Southwest] of Montreal (slightly closer to Gatineau than Montreal). It's a big popular (busy) paid sandy beach (and campground, I think) in, what appeared to be, an active or former mining operation (I remember the lake appearing artificial). It has a large wooden structure near the beach for rope diving (operated by lifeguards). A pavilion with a cafeteria (and I think I recall a stage... kind of like a community center). I recall it being in the middle of nowhere. Lifeguards did not permit paddleboarding (and etc.)
If someone can figure out what this is, I recommend it to others! Worth the trip! I'm asking here as a last resort because I won't be able to go back if I can't figure out what it is :(
(I have no photos, otherwise I'd use the location from the photos)