There's two types of military SONAR: active and passive. Active SONAR is using pings and listening for the echo, but it's not very commonly used by submarines. Passive sonar is just listening out for engine noise from ships and prop noise from other submarines. Warships use active SONAR more because they're already noisy and so if they think an enemy submarine is in the area they'll use it to try and locate, but in peacetime it's mostly reserved for training exercises. It's still terrible for wildlife, even if they don't die it's very distressing and disorientating but it's not constantly scanning like a RADAR.