Seal in the Thames

While I was walking the pup this morning I saw a seal in the Thames just by Blackfriers Bridge. Is this unusual? It was lovely to see.

I saw two in last 2 months (or the same one twice). One just opposite the Oxo Tower, one just down from the Angel. A friend saw one up at Chiswick. I haven't seen one before, not since Sammy who sat on the wooden platform in the muddy creek by Brown's Shad Thames (eye-watering prices for those flats and they look out on all that mud and other horrors?)