This is the inexpensive way to see London from the water. Take a London Water taxi. You can spot them by the Blue Round-all signs that look very similar to the red Round-all signs on Tube (train) stations. The one we got on today was diagonally across from the parliament bldg and the Elizabeth tower which houses Big Ben. The many tourist boat rides that go up and down the river Thames are greatly over priced. This method will save you a good bit of money.
Use your oyster card or electronic payment methods such as Apple or Android pay from your phone or smart watch.