Pam Harvey & Michael Panckridge 
In The Deep End [EPUB ebook] 

The Clued-Up Six are a gang of friends always on the lookout for mystery and adventure. In the Deep End is the second book in the Clued-Up Six series. Something is not right in teasdale. First, Gabby, one of the most talented swimmers in the state, is well beaten in the swimming trials by Andrea taylor. Second, Hannah, the brightest kid in school, misses out on her chance to win the national Whatever, Wherever competition when she loses to her classmate Dave Kelly. Neither Andrea nor Dave posed any threat to the girls before, so who — or what — is helping them to win? Ages 10 — 13