Dark and Lonely Water by Graeme Reynolds

When Samantha Ashlyn is forced to return to her home town to write an article on a series of drownings, she initially resists, finding disturbing similarities to her childhood experiences. However, once she starts looking into the assignment, she finds that things are not what they seem. An ancient evil is rising again, aided by what appears to be a centuries-old conspiracy to keep it hidden. With the help of a disgraced police diver, Sam races to stop the nightmare before more lives are lost. Not realising that her investigation has put herself and those she loves in terrible danger.