I’m excited to see where people take this! 
There have been various interesting ideas discussed previously, like:
- Acoustic beacons for equipment recovery
- Exploring acoustic positioning
- Measuring acoustic attenuation of the environment
- Oceanographic water property sensing
- Basic scanning sonar
- Echosounder distance tracking algorithms
- Anti-synchronisation for obstacle avoidance
but I’m sure there are other ideas floating around out there too, including some that haven’t been thought up yet!