Of all the different types of mysteries, the 'locked room' type seems to be the most baffling. Author Gene Grossman tackled the genre head-on in his 'The Magician's Legacy,' number 7 in the Peter Sharp Legal Mysteries, and here in number 14 of the series he attempts to present it in another form - a boat at sea. In this case, the victim is alone on his boat, half way between Catalina Island and Marina del Rey California when he makes a distress call to the Coast Guard. After his first few words