Fishy mystery. Son of a California marine biologist, Corey is photographing fish for Dad's new book when he first sees the greenish light and giant tail of an unidentifiable ""monster."" It could be this or that, he confides to Carol Walter, summer buddy impressed by the high school senior's wide range of knowledge. The sharks don't bother this scuba duo but Herb and Delbert do -- they're trying to salvage secret gold from a sunken ship and fear that Corey's monster hunt will interfere with their operation. Their booby-trap for Corey finally scares the monster away, but not before he takes a picture. Probably the prehistoric ""dinichthys,"" says Dad. The growing friendship between Corey and Carol is slightly more credible than the uneven scheming of Herb and Delbert, and the underwater observations sometimes swamp the story.