A 30-year-old Australian woman was arrested in Union Square yesterday when she did not heed police officers' warnings that a planned water fight event had been canceled. Cassandra Wu, in town for her 30th birthday, was arrested after spraying the crowd (including the officers) with a water bottle. "This is crazy and this would never happen in Australia," Wu's sister said. The water fight, originally planned to celebrate the beginning of the three-week Street......