Six people charged after 21st birthday cruise brawl at Sydney Harbour

A young NSW woman whose 21st birthday cruise was marred by a brawl at Sydney Harbour says her function was not at fault.

Six people have been charged over the fracas, which erupted about 11pm on Saturday night as Lana Battaglia was celebrating her 21st with friends on board a Constellation Cruises vessel.

It had returned to the dock to offload a "severely intoxicated person" after leaving the wharf at 5.30pm, police said.

According to police, officers were called to the wharf over a separate fight and as the cruise boat was leaving, a number of people noticed a woman in the water who had jumped from the wharf.

She was taken to hospital for assessment.

But police said another fight broke out on board and a number of people were assaulted.