Great barracuda, Sphyraena barracuda.
Why does the great barracuda open and close its mouth constantly while laying stationary in the water? When not swimming the opening and closing of the mouth forces oxygen rich water through the gills. This great barracuda was at the 10-foot level in about 30 feet of water. It remained still while I took photos but opened and closed it mouth constantly. Many divers interpret this behavior as aggressive, but the big guy is just getting his air supply. I pictured that he had just eaten a medium sized grouper and was digesting his meal, happy to hang out, rest and relax.
This one was about 4 feet long with a head twice as big as a human head and long razor-sharp teeth clearly seen when he opened his mouth. They can get to 4.5 feet long and weigh 100 pounds. They are named great for many reasons. Aggressive predators they can reach speed of 36 mph racing after prey.