Dark gold / David Angsten.

Enhanced descriptions from Syndetics:

"Sunken gold, black magic, sea monsters, a beautiful Brazilian in a bikini-what more could you want from a summer thriller?" -Kirkus, starred review   Three college buddies, two ravishing shipmates, a wealthy yachtsman (and plenty of Mexican tequila) combine and combust in this thrilling tale of a search for sunken treasure-and survival against an awesome creature of the sea.  After graduating college, Jack Duran and his two best friends, Rock and Duff, are looking forward to a carefree trip around the globe-a last stab at freedom before tackling the "real world."  But when Jack's vagabond brother, Dan, vanishes in Mexico, they head off instead for Puerto Vallarta on a mission to find him. Following Dan's trail of shady dealings, and fueled by a potent Mexican tequila, they soon join forces with a Bahamian yachtsman-a charismatic eccentric with a mysterious past-and become hopelessly enamored with his two enticing shipmates. Sailing down the jungle-covered coast of Mexico, they discover that Dan was searching for a 19th-century shipwreck reputedly laden with gold. When they finally locate the sunken wreck in the bay of a remote fishing village, they enter itseerie, black waters and begin their hunt for the treasure. But, one by one, the divers begin to appear Jack soon discovers that a mysterious leviathan seems to be guarding the wreck.  Could it be the fabled Diablo Blanco -a deadly local legend come to life?  Blinded by the prospect of untold riches, Jack pursues his harrowing quest to the very brink of the abyss, bringing him finally face-to-face with an awesome, mythical terror. A fast-paced, high-adventure thriller reminiscent of The Deep and The Beach , Dark Gold is the hair-raising tale of a lost ship, a dark obsession, and a terrifying denizen of the deep.   "I sat down with Angsten's thriller in the afternoon, and I was still reading at 2 A.M.  Dark Gold builds to a frantic and explosive climax."-Royce Buckingham, author of Demon Keeper .   "Taut and well-crafted.  I couldn't put it down." - John Scott Shepherd, author of Henry's List of Wrongs and The Dead Father's Guide to Sex & Marriage , and screenwriter for Joe Somebody and Life or Something Like It   "Sunken treasure, exotic locations, adventure and intrigue on the high seas, and a dark and disturbing undercurrent of mystery that H.P. Lovecraft himself would have been proud of are all key ingredients of David Angsten's first class roller-coaster of a novel." - Nick Redfern, author of Three Men Seeking Monsters