I was read a book at school, which I really enjoyed , but can't for the life of me remember the name of it.

I don't remember much, except that a young girl had to enter an underwater world to collect something or maybe rescue someone. She had to swim out to the middle of a lake to fetch some water lilly leaves to make a magic mask that would allow her to breath underwater. Then she went down on her mission, possibly into the same lake. I remember everyone was scared of the pike!

Please help!