Jo Wells, 38, spent at least three minutes under water after the pleasure cruiser's collision with a gravel barge, the Bowbelle.
She thought she was about to die, but surfaced suddenly and emerged from the river with only minor physical injuries. But Ms Wells has endured seven years of trauma since the night when the Marchioness went down with the loss of 51 lives.
The river boat had been hired for a private party, and Ms Wells was on board as a guest of Karen Jarvis, 28, her cousin, who was to die in the disaster.
Ms Wells has suffered flashbacks, severe anxiety and she is unable to concentrate. Doctors have diagnosed that she is suffering from post-traumatic stress disorder, and she is asking the court to award damages against the respective owners of the Marchioness and Bowbelle.
Leading musicians, including Pete Townshend of The Who and her former teachers from the Royal College of Music, are expected to give evidence to the court of her exceptional musical talent and outstanding career prospects. A college examiner will describe Ms Wells as one of the best musicians he ever heard.
She was playing with bands including Tears for Fears, Kissing the Pink and the Communards before the disaster. But she subsequently lost the control of her lip that is essential to players of brass instruments. She is now unable to work, has sold two of her saxophones, and lives on income support.
A jury at a coroner's court in 1995 decided the 51 victims had been "unlawfully killed" when the 1,880-tonne Bowbelle, steaming at four knots, struck the stern of the Marchioness near the central arch of Southwark Bridge on 20 August 1989.
The pleasure boat's upper deck was ripped away; most of the dead were on the dance floor when the collision happened.
Survivors and relatives of victims claimed that those responsible for the disaster should face manslaughter charges for a "hit and run accident" in which the Bowbelle broke maritime rules.
The Crown Prosecution Service decided last year that no criminal charges would be brought, but owners of the two vessels have accepted liability in civil law after action by a group of relatives and survivors. A pounds 6m fund to pay compensation was set up, but insurers for the owners have challenged individual claims for damages, including the action begun by Ms Wells.
"She will not have to prove that she is still suffering from post-traumatic stress disorder," a spokesman for Alexander Harris, her solicitors said. "She will have to prove that it was caused by the disaster.
Ms Wells, from Salford, Greater Manchester, will also have to provide the court with an accurate estimate of the career achievement she would have enjoyed had she not been involved in the disaster.
The Marchioness Action Group has claimed the insurers should have settled quickly, but have chosen instead to quibble, submitting some survivors to repeated psychiatric examination.
Relatives of the dead have been asked for "petty" details to support their claims, including a mother who was asked to prove the hourly rate her daughter had been paid for seven hours' work a week as a domestic.