The Colossus crisis - Katherine Roberts

ADVENTURE AND MAGIC IN THE ANCIENT WORLD
For years the titanic figure of the Colossus guarded the harbour entrance on the island of Rhodes and kept watch over its people. But a fatal prophecy foretells doom and destruction.

When an earthquake strikes and the Colossus of Rhodes collapses, humans and an ancient race of sea creatures - the telchines - come face to face. At the centre of the struggle is a living Gift from the gods with magical powers for good and ill. A girl sponge diver, half-human and half-telchine, must restore the Gift and reveal an ancient prophecy, but she herself is struggling to come to terms with her mixed heritage.
Snatched from her home by those intent on securing the Gift's powers, she is befriended by a trainee priestess and a gang of robber children, and with them battles with corrupt priests and scheming Councillors.
But what is the strange Gift? Who can she trust? And which race will she ultimately side with - human or telchine?