I'm sure it's crustaceans etc, because when you sail over a shallow reef in light winds you can hear the sounds increase.
From the net;
"Crustaceans make noise for defensive and, possibly, courtship purposes, but the unknowns far outweigh the data. A variety of crabs, lobsters, shrimps and other crustaceans have developed noise-making capabilities as diverse as those employed by terrestrial insects.
The crackling made by barnacles as they open and close and move their articulated appendages can be detected for miles. The predominant sound coming from coral reefs is the incredibly loud popping of tiny bubbles (cavitation) generated by hordes of small snapping shrimp for hunting and communication. Mussels can produce sound by stretching and breaking the byssal threads that attach them to the substrate. Urchins have been observed making crackling noises by clicking their sharp spines as they move. This crackling can also be caused by the urchin's test (the shell surrounding its body cavity) rubbing against its Aristotle's lantern (feeding apparatus)."
One tiny shrimp is one of the noisiest animals around;
"Divers have witnessed the ceaseless cacophony that colonies of these shrimps produce; their loud crackling sound has been described as if a hundred people are cracking their knuckles at once, or like sizzling of fat in a pan or like 100 people bursting bubble wrap at once!"
www.todayifoundout.com/index.php/2010/09/a-shrimp-is-one-of-the-loudest-animals-on-the-planet/
