I had one couple of years ago it was at topcliffe, middle of the night I was woke by a sneezing noise from the river. I got up and turned my head torch on and then there was an allmighty splash and bow wave. Totally shocked I slept with 1 eye open that night. It hung about most of the night. In the morning the bailiff told me the seal beached itself at night where my bivvy was.
Another time I was at Tadcaster must have been around 2000 I was down by mucky hole and one popped it's head up, then disappeared. My mate I was stood next to missed it - I said to him did you just see that seal?