As compared to the tour busses, the duck tour was a bore, except for the novelty of going from the street into the water. Admittedly, I am an American who enjoys learning history and tidbits of information. Our guide provided little of that. Indeed, she spent more time off the microphone trying to talk to the driver, or to passengers about where they were from. She merely pointed to buildings and gave the name without telling any stories about the place. It may be that we simply had an unusually bad guide, but neiither the children nor the adults enjoyed the tour much. The boat does not go down as far as the Tower.
If you simply want a ride on the Themes, try going to the London Eye, and then taking their brief water taxi to the Tower.

My first tip with the Duck tour would be to see if you could get and half price vouchers. I did, I got a 2 for 1 package that translates into great value. You can book your ticket one of two ways - either from there website which is very nice and simple to navigate or to go to there office on the square just behind the Millennium wheel. If you do have a voucher you will have to go to there office.

The duck Tour departs from just outside there office. It goes around all the landmarks including Parliament, Downing Street, Nelsons Column, Buckingham Palace and Westminster Abbey. After a 40 Minute tour the Vehicle will then go into the river for a short cruise. The journey is accompanied with a brief history in a light-hearted manner by one of the Duck Tour team.
For the price is was great and highly recommended.