Climate Change Is Affecting The River Thames,

Good News ’ Seals, seahorses and eels are among the species now residing in our river, along with, surprisingly, tope, starry smooth-hound and spurdog species of shark.’

Video Seahorses found living in River Thames in London - BBC News - YouTube
There are a few Observations of Syngnathidae (Family) in iSpot see mine with eggs

in the old days, I spent much of my time relaxing beside a rock pool at Fish Hoek, there seemed to be some species of fish that we only saw occasionally, and just once, I saw a seahorse which I thought might have been brought down by the current, following a raging storm up the east coast.
Some famous wrecks MTS Oceanos - Wikipedia
And of course the SS Waratah - Wikipedia