As a soon to be certified lifeguard...
Lifeguarding is entirely visual. The signs of a drowning victim do not include screaming (if your underwater or thrashing around in it you aren't likely to get many sounds in). Lifeguards are trained to "look" for victims, not "hear" them.
In fact, it is easy to become distracted by loud noises when they are scanning the area. That's why it may be better to be deaf, less chance of being distracted.
