You are eleven years old, and you are also a boy without fear,"" Marco reminds himself more than once during this jaunty, elastic edge-of-the-chair adventure. With good friend and good sport Sally and her hound Happy (a stumblebum as a sleuth), he's in pursuit of the mysterious pair who've hired deep-sea diver Larry Larue to find cannonballs in sunken Fort Beauregard. Locating the fort involved a ghostly search in a newspaper morgue, and there's a real headless ghost on the parapet--who turns into (""How did you know my name?"") Professor Jacob Q. Finger, erstwhile Civil War scholar become unscrupulous treasure hunter. Mr. Foster is also something of an authority (Rebel Sea Raider) on naval action around New Orleans and he gives the gold-cored cannonballs considerable punch. But his chief asset is sharp-witted, level-headed, fair-minded Marco, saluted even by the intercepted Finger as ""a boy after my own heart."" Macro and the Tiger was a sport among pet stories; this is a mitzva among mysteries.