Event    From 3/9/1878  To 3/9/1878

London's worst peacetime disaster.
The Princess Alice was a passenger paddle steamer, making what was called a 'moonlight trip', from Swan Pier near London Bridge to the former Rosherville Pleasure Gardens in Gravesend.
At about 7.40 p.m. on her return journey, she was preparing to disembark passengers at Woolwich. At the same time, the coal ship S.S. Bywell Castle was coming down river in the opposite direction. Her master became aware of the steamer, and set a course to pass it to starboard. The Princess Alice was labouring against the tide, and her master followed the normal practice of seeking the quieter water on the south side and altered her course to port, bringing her into the path of the Bywell Castle.
Although the engines of the Bywell Castle were slammed into reverse, it was too late and the steamer was torn in half, sinking in just four minutes.
Many passengers couldn't swim, but the death rate was almost certainly increased by the fact that the twice-daily release of 75 million gallons of raw sewage from outfalls at Barking and Crossness had occurred just one hour before the collision. The inquest suggested that many deaths were caused by severe vomiting and subsequent dehydration after ingesting the filthy water
No records were kept at the time, but it was estimated that about 900 passengers were on board. Of these, between 650 and 700 were believed to have died.
A lot of the passengers were below decks in the saloon, and the speed of the sinking gave no chance of escape. In fact, when the wreck was raised, many of those who drowned were still crushed together standing upright.
The masters of both ships were deemed to have had equal responsibility for the accident.
To a lesser extent, history repeated itself on the Thames, when in August 1989, 51 people were drowned when the pleasure steamer The Marchioness was rammed by the 'Bowbelle' dredger.

Londonist provides a link to a 5-minute documentary film on this, Britain's worst transport disaster.