FAR TORTUGA
by PETER MATTHIESSEN 408 pages. Random House. $10.95.
Captain Raib Avers and eight ragged black and mulatto crewmen set out from Grand Cayman Island to hunt turtles in the southwest Caribbean. Their ship is the Lillias Eden, a once proud schooner now yoked to brand-new twin diesel engines in its converted cargo hold. Avers' legendary temper is even blacker than usual. Though it is late in the turtle season, he needs a good catch to pay for the overhaul of his ship. He rashly refuses to worry about Eden's lack of a chronometer, life jackets, fire extinguishers, or a radio that...