Most balls used in sport will not last too long if over exposed to water but, with water polo balls they are designed to be used in water (hence the name). Care should be taken though to ensure you get maximum use out of your ball. Water polo balls have a unique design that when the ball gets wet, the grip is stronger.
Although they are designed to with stand wet conditions, you should never store the ball away when still wet. After use wipe it down with a dry cloth or towel and store in a cage or ball bag with vented sides.
Do not store the ball indirect sunlight as the grip and pressure of the ball can be seriously affected.Water Polo Ball Buying Guide