Thames Clipper. Oysterising the Thames - The River Concordat

http://www.thamesclippers.com/routes-times-prices-booking/routes-times-prices-booking.html

This regular river service can be a bit low key in its presence at the piers in tourist areas, presumably so the tourist oriented boat operators can catch the tourists. Thames Clipper's all day ticket is £12 or £8 if you have a travel card.

http://www.thamesclippers.com/main/news-15.html

This press release from 6 April ('Oysterising the Thames - The River Concordat') says "From November this year Thames Clipper services will be able to accept Oyster Pay as you Go"