Thames Clippers have the longest boat routes on the Thames -- all the way from Putney to North Greenwich (The O2). That's a 15-mile stretch that takes you through the whole of central London.
Technically the boats are aimed at commuters rather than tourists. They are very fast, stop at a lot of places, and don't have as many outside seats as some of the other London boats, but it's still a very enjoyable way to travel up and down the river.
They stop everywhere from at Chelsea, London Eye, Embankment (for Trafalgar Square), Blackfriars (for Temple), Bankside (for Tate Modern and the Globe), London Bridge (for The Shard), Tower Millennium (for the Tower of London and Tower Bridge), Canary Wharf, Greenwich (for the Cutty Sark and Royal Observatory) and North Greenwich (for The O2).
Unfortunately there is no one boat that goes the entire way -- if you want to enjoy the entire route then you have to change boats when you reach the Embankment or London Eye. But don't let that put you off -- you can almost treat it like the tube: buying a day pass and using it to go from Westminster, to The City, and up to the Tower of London and Tower Bridge.
Another good thing about this boat is that you can use an Oyster card, Visitor Oyster card and contactless card on it. They have those yellow readers on the pier (the exact same ones they have at train stations), so you just tap down like normal. You can buy a Visitor Oyster Card online before you go. You can also combine them with a London Pass, to save you even more money.
Craig has been on this boat himself, and has written a big review including lots of photos of what you can see out the window. (You might like to read his previous review about it as well.) Feel free to ask him a question about the Thames Clipper.
There are plenty of other boats that go up and down the Thames, if you don't fancy this one. The two best ones are run by City CruisesTRS River Cruises. Craig has written reviews of those ones as well: City Cruises review and TRS River Cruise review. The big advantage with those two, over the Thames Clipper, is that they have lots more outside seats and they usually provide a bit of amusing commentary onboard as well.
If you fancy something a bit more exciting (eg: more terrifying), then how about riding on a speed boat instead? Thames Rockets run speed boats up the river which will really get your heart pumping.
Guest – “Are OAP Freedom Passes usable, please?”
Admin – “You get 50% off with a freedom pass. That doesn't apply to all ticket types, though.
More details can be found about it here: thamesclippers.com/route-time-table/prices”
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Craig’s review – “Some days you just want to sit in a boat until the river runs dry... have it keep floating until there's nothing left to float on. That's why God invented these Thames Clippers. These boats have got the longest routes in London: from Putney all the way up to North Greenwich and O2 Arena. But here's a word of warning: they're not sightseeing boats. They don't have a to… continued”
Read Craig’s review of this event