I don't know at what frequency the Garmin operates, but having worked on our own transmitter and in the past been familiar with the Liquivision transmitters as well as some ultrasonic comm systems, and knowing how noisy scuba divers & regulators are in general, I expect it's like someone having a quiet conversation in a full football stadium. You can hear if you are close enough and listen. I'm certain that the sound output is a very tiny fraction of, for instance, a boat's depth sounder. I wouldn't worry too much about disturbing creatures.
The main advantage of acoustic signals is that underwater they have the potential to travel longer distances, so you can monitor other divers. Our own transmitter (not yet on the market) we get 2-300 feet reception with decent consistency, and I can only hear it if I try hard. But then, I'm old, and my high frequency hearing is probably not great.
Ron