Loch (The Zone Unknown) By Paul Zindel

Loch and his sister are with their father on a scientific expedition to track lake monsters. Their father's boss, Anthony Cavenger, a ruthless publishing mogul, is determined to prove that the legends are fact. Until now, it has been a fruitless exercise. But suddenly, on a routine exploration, a hideous water beast explodes out of the water, and a photographer, hoping to get the picture of a lifetime, loses his life instead. The plesiosaurs terrorize the secluded lake community, but Loch encounters a baby plesiosaur and realizes that the monsters only attack when threatened. So he risks his life-and the lives of his family and friends-to save the prehistoric creatures from destruction