Anyone know this children's book?

I need to remember the name of an old children's book - publication date had to have been sometime in the seventies. It started out 'It was a dark night...' (vague I know). It was a story about a diver who was looking for a buried treasure. The illustrations showed the diver in old metal scuba gear. At one point the diver is approached by a shark, but he's able to scare the shark away using bubbles from his oxygen tank. Can anyone help me?!? Thanks.