With all the splashing that goes on in water polo, it makes sense that the players would wear caps to prevent the water from clogging their ears. But if you take more than a cursory glance at the gear, you’ll realize there are tons of holes in the earpieces. So if they're not keeping water out, what purpose do those bonnets really serve?
The primary reason is for injury prevention. The plastic guards over the players's ears—which regular swim caps don’t have—help absorb the impact of the ball or a body part that so it doesn’t result in a ruptured eardrum or other damage. A rupture “typically heals on its own with time,” sports medicine physician Dr. David A. Wang wrote for the Hospital for Special Surgery’s blog, “but given the possible damage to the inner ear from water, the athlete will often have to remain out of the pool until healing occurs.” Considering how tightly packed the schedule is for Olympic water polo games, the players don’t exactly have the luxury of sitting poolside for a few weeks while their eardrums heal.
Since the head is the only part of a player’s body that consistently stays above the water, the bonnets also basically function as the athletes’ jerseys, allowing them to distinguish teammates from opponents (and also between teammates, as their numbers are printed on their caps, too). One team usually wears dark-colored caps, the other team light-colored caps, while the goalkeepers are in red.