If you want to get a look at the ducks, there are usually a couple of them sitting at the place where the tours start, which is just behind the gardens at the back of the London Eye. I'm a native and a river fiend, but I've never been on one - driving into the river has always appealed, but 75 mins with no stops in a very cramped thing with lots of over-excited kids, not so much. The Blitz tour might be cool, though.