Open circuit scuba gear is noisy because you are regularly exhaling gas. The regulator makes a little bit of noise, but the exhalation is generally what spooks

Open circuit scuba gear is noisy because you are regularly exhaling gas. The regulator makes a little bit of noise, but the exhalation is generally what spooks animals. Lacking the need to exhale/vent gas (very often), Closed circuit rebreathers are extremely quiet. Some serious underwater photographers moved from the much cheaper (and arguably much safer) open circuit setups to rebreathers entirely to avoid spooking subjects. A rebreather probably can't quite compete with a freediver for silence and agility, though would certainly have a lot more time underwater to make up for it. At least some organized whale shark dives are freedive-only, probably partly to protect the animals and partly due to the time and noise required to put scuba divers into the water.