It sounds like the biggest problem in your friends unfortunate drowning incident was not recognizing a potential drowning victim per se, but in seeing him while he was being obscured by other swimmers in the area. I would assume anyone frantically splashing, otherwise struggling, or motionless in a pool might be drowning and investigate. Of course kids do these exact things when they're playing around and not drowning, but they should be investigated anyway to be on the safe side. But not being able to see all the swimmers in your pool because of obstacles or overcrowding needs to be addressed by hiring more lifeguards and positioning them appropriately, or limiting the number of people in your pool to a safe number.