The 16-year-old was rushed to St George Hospital after two boats collided at Grays Point in a horror start to the Australia Day long weekend.
Emergency services were called to the Grays Point boat ramp shortly after 10.15am on Friday where they found two tinnies had collided, NSW Police said in a statement.
Paramedics treated the girl at the scene before she was taken to St George Hospital in a critical condition, where she later died.Â
Two teenage boys, both 16, were also taken to St George Hospital for mandatory testing.Police Marine Area Command is investigating the incident.
January 26 is one of the busiest days on Australia's beaches.
At least 12 people were rescued from rips across beaches in three separate incidents on the NSW south coast on Friday.
Austinmer Beach in Wollongong was closed earlier in the day after a man and a group of teenagers became stuck in a rip on opposite ends of the beach during unpatrolled hours.
Another group of men was rescued from nearby Woonona Beach after being caught in a rip while swimming outside the red and yellow flags.
Surf Life Saving NSW cautioned those who plan to hit the beach that drowning risk climbs by four times on Australia Day.
"The statistics show that Australia Day is one of the busiest days of the year on our beaches and is always a big day for our volunteer surf lifesavers and lifeguards," Surf Life Saving NSW CEO Steven Pearce said.
"Enjoy the weekend but make sure that if you're heading to the beach, visit a patrolled location and only swim between the red and yellow flags."
At least 11 people have already drowned on the NSW coast since December 1.
All drownings occurred at an unpatrolled location, or outside of patrol hours and the red and yellow flags.
The warning comes after a nurse, two students and a woman on holiday from India died in one of Victoria's worst drowning incidents when they were caught in a rip at Forrest Caves Beach on Phillip Island on Wednesday.
NSW Emergency Services Minister Jihad Dib said the dangers of swimming at unpatrolled and remote locations have been tragically highlighted across the summer.
"Surf lifesavers are anticipating an extreme level of activity over the coming days, so we are asking everyone to be water safety conscious and support our volunteers by listening to their advice," he said.
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