The clue and the characters belong to JK Rowling. Other bits belong to Damon Knight and CBS. The plot of this drabble belongs to me.

-?-!-?-

Come seek us where our voices sound,
We cannot sing above the ground,
And while you're searching ponder this;
We've taken what you'll sorely miss,
An hour long you'll have to look,
And to recover what we took,
But past an hour, the prospect's black,
Too late it's gone, it won't come back.

"What does it mean, it's gone?" asked Cedric.

Dumbledore coughed, embarrassed, and said "Well, the merfolk do require payment for their services and historically...".

Handing the champions a book, he said "You might want to check the chapter 'Underwater Preparation'". The book's title read 'To Serve Man'.