Read this book in the late 1970s, but think it was written maybe in the 50s.  It was set after WWII, about an ex Navy LTJG who was salvage diving, came across a young stowaway and put the boy to work for him on his boat.  It was an adventure story, with a run-in with an octopus, storms and bad guys, but in the end they found the treasure.  Ring any bells?

Many thanks!