The watch makes this a good story; swimming up the Thames isn't that extraordinary. The beluga (a whale living in the Arctic) nicknamed Moby Dick that swam 300 km up the Rhine certainly bests that:

"On May 18, 1966, a few Rhine skippers near Duisburg reported a white whale in the Rhine to the water police. They reacted by first making the mariners take a blood alcohol test, which came up negative"

(https://en.m.wikipedia.org/wiki/Moby_Dick_(Rhine). Video at https://www.youtube.com/watch?v=JFBGhB-APMs. This beluga apparently was on board a ship that sank while carrying it to England)

I feel like there's a Doctor Who joke to be made out of this.

I feel like if this happened on the other side of the Atlantic, there'd be a punchline along the lines of, "So that's what happened to the alligators!"

Depending on where in the Thames the shark was caught, it was more likely unwell due to being in freshwater instead of saltwater