Outside of the Olympics, you don't get many chances to watch water polo, so if you're tuning in for the first time you might be wondering why the players are wearing headgear. There doesn't seem to be a direct benefit, but there are actually many purposes.
SAFETY
The main purpose for the bonnet-looking caps is safety. If there were no headgear, the players' ears would be exposed to high-velocity thrown balls. Even with the caps, there's danger for those who play. Hearing Rehab Center lists water polo as one of the sports that causes the most hearing damage.