Water Pong is the generic term referring to a revolutionary take on beer pong. The game Poolside Pong is the Inflatable beer pong table that allows you to play beer pong in water. Hence the term water pong.
For lovers of the sport of beer pong (or beirut, however you prefer), water pong is the equivalent, but it can be played in a context in which alcoholic beverages would be inappropriate. The rules of this game are the same as the rules of regular beer pong.
Yo Justine, I don't have any work to do, let's go to the room next to the library and play some water pong.