The Merlin Effect (Lost Years of Merlin)

Kate Gordon has traveled all the way to San Lazaro in Baja California ot help her fathr locate the sunken remains of the "Resurrection"--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 more valuable than anything Kate could ever imagined: a magical drinking horn from Arthurian times wihch--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 cratures, Kate must battle ancient forces to save her father's life...and her own.