JIM CARROLL.
Jim,
    There is no record of the Woolwich Free Ferry ever having suffered a fatal collision or sinking but you could be thinking of the ship, "Princess Alice", a pleasure steamer from London Bridge to Sheerness that was sunk with the loss of many lives, certain of those dying as a result of the polluted Thames water as you described. There is also "The Northfleet", sunk following a collision with a Spanish vessel off Dungeoness resulting in huge loss of life. We have a song in our repertoire and sung by others known as "The Wreck of the Northfleet".
    As youngsters we would travel back and forth on the Woolwich Free Ferry just to watch those enormous engines perform, the engine room being open for public viewing through large , clear glass panels and the smell of the lubricating oil is still evocative.

John