Does the Thames River in London have dangerous animals?

I'm surprised nobody has mentioned seals. Grey seals and harbour seals are regularly spotted in the Thames and in places they can only have possibly reached by swimming up the river. I saw one myself a few years ago in Millwall Inner Dock on the Isle of Dogs. read more

The most dangerous animals you’re likely to find in the Thames, at least as far downriver as London, is bacteria. Far more deadly than any fish, and I strongly don’t recommend attempting to swim in there. Further upriver where it’s cleaner, the most dangerous animal you’re likely to encounter is an otter. read more

The River Thames flows through London and past some well known landmarks. There are boat trips up and down the river within London and most tourists visiting the capital take a trip on the river so I suppose that would make it quite popular. read more