Although you can play water volleyball with a variety of different types of balls, from beach balls to basketballs, and soccer balls to volleyballs, your best bet is to get a purpose built water volleyball ball. Most other types of balls (for example, soccer balls and traditional indoor volleyballs) are not waterproof and will become saturated with water over time.
A good water volleyball is always waterproof. It will be able to withstand the rigors of chlorinated pool water or lake water. Most balls are coated with vinyl or plastic to make them waterproof. Some water volleyball balls have a soft coating on the outside to make the ball easier to grip and less painful when the ball hits bare skin (like an open palm or someone unlucky enough to get hit with a spiked ball).