Don't think those teenagers will be swimming at night again for a while
That's creepy. Maybe it's possible not to notice if it's in the deep end, but I guess it's more likely that most people either glimpsed her briefly and assumed she was alive, or were so put off by the encounter that they felt too uncomfortable to report it. Diffusion of responsibility - if something distressing happens in a crowded place, people will assume that other people will deal with it and save them the trouble.