If the water really was that murky, then the story is only as unbelievable as the part where the kid told the lifeguard and no one did anything.
In murky water, the lifeguards should have been on heightened awareness around the slide / deep water. One person goes down the slide, one person comes up. In the story, the vicitm and the boy went down the slide together. I am sure this is against the rules. The guard was probably distracted, checking someone else, etc. Didn't notice that two people exited the slide, heard the splash, looked over and sees a little boy come up and swim to the side. One splash, one kid. All is well. . .