I've a recollection of a film I watched as a kid (vastly under-aged). It was more than likely 1980s in origin, or at the very latest very early 1990s. It was almost certainly horror.
A lot of the film was set around a big swimming pool, not a house pool, but a public pool. The pool had a lot of steam/smoke over it. I'm pretty sure there was some sort of monster in the pool, but it may have been able to move around as well, I'm not sure.
There was a scene that involved tampering with a CCTV camera. (Possibly someone trying to get to the monster without it knowing?) It was a very dark and atmospheric film, very low lighting, low budget etc.
Google searches have thrown up TerrorVision but I'm pretty sure this wasn't it as that was set in a house, and the pool was a small one.