Seabird identification

Can I say a big thank you to all of you who help novices like me expand my knowledge of birds. This is such a useful tool. 

Walking today at Hartlepool Marina, I spotted a bird I had no idea about. Unfortunately I didn't have my camera handy, but I did get a good look at it. 

Size - similar to that of a tern, perhaps...

Colour - completely black on topside/completely white on underside. The feathers seemed very downy. 

Legs - black

Bill - black

It was swimming on its own.

The close bird I have seen to it is a little auk, but still unsure. Any help would be gratefully appreciated!