The scoring of water polo is most similar to the scoring of

Hockey.

I suppose one could list other sports where the object of the game is to put the ball into the other team's goal, and doing so scores 1 point (one goal) -- and most goals win.  Soccer is like that too.  But scoring in water polo is most similar to that in hockey because much of the scoring in either sport occurs during "power play" situations.  In either hockey or water polo, players can be sidelined for a certain amount of time for aggressive fouls.  The team must play a person short during that penalty time, and the team with the advantage of more players still in the game has a "power play" opportunity, during which goals often are scored.

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