Dark Waters by Catherine MacPhail
Col McCann is used to being in trouble. It is always the McCann family, in particular Mungo, Col's brother, who the police think of first. But Col has recently acquired a new fan - Dominic. Col saves Dominic from drowning in the local loch and discovers what it is like to be a local hero. But Col sees something in the loch, something that leads him to a devastating truth about his brother ...and brings his loyalty to his family and his need to do what is right into direct conflict.