As HugoNissa alluded to, you can't always get a shot as good as his of this tag. Might need him to confirm that it's the right location. Here's proof that I did at least try to get one in exactly the same spot:
Haha, I went into the exact same problem, with mud up to my anckles, before finding out that next to the boat, you have a path made of stones on which you can hop from one to another, bringing you down to the middle of the Thames nice and clean.
Took me ages to clean myself up !