On my my last diving trip I heard a sound underwater that I never heard before. Usually all you hear is your regulator as you inhale air and blow it out and make bubbles, or a boats engine on the surface as its passing by, but this time I kept hearing these "pops" every once in awhile. They were a little muffled because of the water, but very distinct. Then it dawned on me what it was. Dont mind the guys over my shoulder in the background, they're just spear fishing. Hint hint.
I thought you were talking about the ticks I used to hear them all the time when diving or snorkeling always thought they were the fish making noise.