Gulliver Queen, nephew of the famous detective, Ellery Queen, doesn't want to be a detective. But when Gully and his new friend, Fisty Jones investigate the strange underwater noises in the Hudson River, they suddenly find themselves deep in a mystery that Gully can't resist tackling. The menacing tattooed man, the weird one-eyed figure in the abandoned building, the strange but merry Magnus Merlin, magician extraordinare - all add up to an exciting tale of danger and suspense on, and under the waterfront on New York. 

Gulliver Queen had looked forward to spending his vacation with his famous detective-uncle--but things were happening too fast for him now. When his uncle Ellery is unexpectedly called away, Gully is left in charge of gathering information. Suddenly Fisty Jones bursts into the apartment demanding help. Gully rushes off to investigate, and finds himself helpless in a muddle of confusion clues.... First there’s the trick dog. Then the wailing ghost...and a terrible tattooed man. And when Gully learns the strange secret of the space monster, he finds himself facing the worst danger of his young life....(cover)

The french and (propably) dutch translations have no original titles indicated but after obtaining copies we were able to concluded the true 'origins' of the two 'Gully' translations. After this given the cover of the Italian edition it's origin was no brainteaser... There is probably a German edition in existence.
 

  The Mystery of the Merry Magician  Translations: 
Dutch: Het geheim van Magnus Merlin 
French: Les Diamants sous la mer 
Italian: Il mago burlone