In the Bangor Swimming Pool, North Wales, UK they have a camera system setup to detect movement called the Poseidon. It compares the images it receives to the ones in its database of images of swimmers in trouble.
Once a match occurs the lifeguards are paged with the exact location of the swimmer. Recently, this system saved a 10 year old girl that had sunk to the bottom. Within 10 seconds the lifeguards were notified and the girl rescued.
The girl has made a full recovery, and probably owes her life to the Poseidon system. Since she had just jumped into the pool and sunk to the bottom without splashing or screaming for help, the lifeguards probably would have not noticed her.