A number of boat tours and ferries operate on the Thames.
Below is a selection of the trips you can take.

Catamaran Cruises offer tours with commentary,
leaving from Waterloo and the Embankment. Waterloo: from 10.45
until 17.45 every hour (except 14.45) Embankment: from 10.15 until
20.15

Westminster: from 14.30 until 21.30 every hour

Circular Cruises offer return trips from Westminster
Pier (Victoria Embankment) as far as St. Katherine's Pier in the Docklands.
Cruises depart every 30-80 minutes with an option of getting off at London
Bridge Pier. You can also take boats the other direction to Hampton Court,
Kew, and Richmond.

City Cruisers offer a range of trips, including
service between London Bridge and Westminster Pier, and a Pool of London
hop-on, hop-off shuttle, calling at St. Katherine's Pier, Butler's
Wharf, HMS Belfast, London Bridge City Pier, and Tower Pier.

Note: with commentaried cruises the crew will
pass a hat at the end of the trip and you will be expected to toss in
a quid or two as a tip, so make sure you have some change!

Source: Britain
Express