The ball is filled with a saline solution, which makes it heavier than normal water and slowly sinks instead of rising to the surface. This allows strong players to throw and pass it for a few meters, despite the drag of the water. There aren’t many rules in this sport, players have to stop opponents from scoring, at any costs. It’s even permitted to sit in the basket to block a player from scoring, but if you can’t hold your breath long enough, you’ll have to go up for air and leave the “goal” exposed.
Check out the short video at the bottom, to get an idea of how underwater rugby is played.