Something's Down There by Mickey Spillane
To the casual observer, Mako Hooker looks like any other grizzled fishing-boat captain trawling the Bermuda Triangle, but he's hardly your typical fisherman. Hooker is in fact a retired government operative taking a much-needed respite from his highly secret, highly lethal career in the States. But when local fisherman begin to fall prey to a mysterious sea creature the islanders dub the eater, he discovers the truth in that old saw about the spy game: You're not retired from the Company until you're dead. Is the monster a prehistoric beast rising from the depths? Or mines from a sunken WWII destroyer? Or the work of someone with an agenda even more deeply undercover than Hooker's? As he moves steadily closer to the truth, Hooker realises that someone (or something) is plotting to stop him, and only his rusty instincts will save him this time.