I actually had a similar experience when I was a child. Fortunately the water wasn't that deep and although I was also having the same "response" I managed to get out of there in time by treading the bottom of the pool. It was indeed quite traumatic in the sense that nobody nearby seemed to be noticing it occuring, including my parents. Maybe communication in my family had always been a difficult matter so I didn't even really mention that to my parents afterwards I guess. Safety measures should definitely be paid more rigorous attention to.