The first one I think is easy ... the Thames is tidal and the variance between high and low tide is quite a bit (over 20' - and that can be altered further by the Thames barrier further up river). Notice the line of boats that are tied up in what is seemingly the middle of the river. Those boats are tied to submerged moorings or pilings. This guy is walking on partially submerged pilings along the tide line of the river.
The second two I have no clue. They were pretty clever.