The Merlin Effect — T.A. Barron

Kate Gordon has traveled all the way to San Lazaro in Baja California to help her father locate the sunken remains of the Resurrecion--a galleon wrecked in 1547 and believed to cradle in its belly a fortune in gold and silver. But as Kate is about to discover, there lays entombed in the dark and treacherous waters of San Lazaro a treasure far move valuable than anything Kate could ever have imagined: a magi-cal drinking horn from Arthurian times which--as legend would have it--led to the demise of the great wizard Merlin.
Drawn by the power of the horn into an undersea world inhabited by strange and terrifying creatures, Kate must battle ancient forces to save her father's life...and her own.