Mystery bird - please help!

Hello everyone! I'm new to this community.

I saw this little thing yesterday and am yet to identify it, despite lots of searching through books and online. It was flying above the water like swallows or martins might, but it looked like a swift in its shape, though in size it was a bit smaller. It did not swoop down to drink. Once it landed, it didn't move much. At no point did it make any noise, as far as I know. It seems to have a white circle around its eye. What could this thing be? Is it a baby of something?

Sorry for the quality of the photos - it was quite small and across the river.

Thanks in advance for your help.