Whatever happened in Winklesea? - Helen Cresswell

Dan and Mary Kane's Gift from Winklesea was their closest friend, and practically a member of the family. So now that he's disappeared, they miss him terribly. Dan in particular pines for the odd, sleek, little grey creature that brought so much fun into their lives. On Dan's birthday, all he wants is to go back to Winklesea. Maybe, just maybe, they'll find the Gift again. But there are people who are looking for the Gift, too - but for different reasons. They want to lock the Gift up, as a freak, in the local Acquatic museum. Dan and Mary know that all the Gift wants is to be left in peace to play on the beach, but how can they convince everybody else?