At least 30 people died when a pleasure cruiser, packed with young party-goers, and a barge collided on the River Thames.
The captain and second mate of the barge, the dredger Bowbelle, are now under arrest.
Among those still missing are the captain of the cruiser, the Marchioness, and a city banker who chartered the boat to celebrate his birthday.
There are fears the final death toll could be as high as 60.
A coastguard placed 51 roses on the Thames to mark the 20th anniversary of the Marchioness disaster, the sinking of a pleasure boat in which 51 died.
The names of the dead were also read at a lunchtime service by one survivor, 62-year-old Odette Penwarden.
More than 130 partygoers were on the Marchioness when it collided with a dredger, the Bowbelle, and sank near Southwark Bridge on 20 August 1989.
A Marine Accident Investigation report blamed poor look-outs on both vessels.
In the collision the anchor of the Bowbelle first cut through the side of the Marchioness. The Marchioness then rolled over and quickly filled with water while being pushed under by the Bowbelle. As the ship capsized the entire superstructure of the Marchioness became detached. The formal investigation puts the time from the collision at 1.46 a.m. to complete immersion at close to 30 seconds. Witnesses quoted in the formal investigation describe the Bowbelle "hitting it [the Marchioness] in about its centre then mounted it, pushing it under the water like a toy boat." Of the deceased, 24 were recovered still in the wreck. The majority of the survivors had been on the upper decks at the time of the collision.
The disaster was found by the Marine Accident Investigation Branch to have been caused by the poor visibility from each ship's wheelhouse, the fact that both vessels were using the centre of the river, and the lack of clear instructions to the lookout at the bow of the Bowbelle. In 1991, the skipper of the Bowbelle, Douglas Henderson, was tried for failing to keep a proper look-out but after two juries were deadlocked he was formally acquitted. A Coroner's inquest on 7 April 1995 found the victims had been unlawfully killed.
Following pressure from the Marchioness Action Group, whose publicity front had been handled by photographer and party attendee Ian Philpott on 14 February 2000, John Prescott as Secretary of State for the Environment, Transport and the Regions ordered a Formal Investigation into the circumstances of the collision, to be chaired by Lord Justice Clarke. Lord Clarke's report also blamed poor lookouts on both vessels for the collision, and criticised the owners and managers of both vessels for failing to properly instruct and monitor their crews.
In 2001 an inquiry into the competency and behaviour of Captain Henderson by the Maritime and Coastguard Agency concluded that he should be allowed to keep his master's certificate as he met all the service and medical fitness requirements. However, they "strongly deprecated" his conduct in drinking 5 pints of lager in the afternoon prior to the accident and for his admission that he had forged some signatures on certificates and testimonials in order to obtain his master mariner certificate of competency in 1988.