Goliaths are certainly large and intimidating creatures. Coming face-to-face with one the size of 55-gallon drum for the first time is an experience few divers forget.
Ive been the water with them hundreds of times in 25 plus years diving (from Ft. Meyers down through the southern Gulf of Mexico on up the east coast to the Stuart) and never had a problem from one.
The booming sound you hear coming from is created by the muscles flexing their swim bladder, which they produce in rapid succession two to three times in a row. Up close, the action literally causes their body to shudder. And yes, it is a threat display. In grouper speak, the message is youre in my territory, go away! But so typical of these animals, its more bark rather than bite. The moment their bluff is called, the big fish generally back, sometimes opting for the safety of a deep hole in a reef or wreck.
When they do that, my word of warning is this -
Do not block the exit! Once they feel their safety inside their hole is gone, they will often try to flee, blow out the first opening they see, slamming into what ever dumb sap happens to be blocking their escape route.
Walt Stearns
Underwater Journal