DUKWS were amphibious troop transport vehicles used in World War II, and a number of companies around the world (or maybe they're all one company) have hit on the idea of using them to conduct city tours.
London's no different. London Duck Tours offer 75 minute tours around central London, which includes a 30 minute section spent in 'amphibious mode', cruising up the Thames (not for very far though). Departing from Chichley Street, just behind the London Eye, you go over Westminster Bridge, around Parliament Square, through the west edge of the West End, up to Buckingham Palace, and back -- see their site for a more detailed route map.
Nik Clayton writes: I took the family on one of these tours one Saturday in January, and can recommend it. The tour guide was amusing, talked non-stop, and had a ready supply of information and factoids which kept the children amused. And there's the occasional harmless innuendo to make sure the adults have a laugh too. One note though -- you're effectively outside the entire time, so make sure you wrap up warm.