@Joe_Haase
Could you provide the parameters you used to set up differential thrust? I inherited a similar project. I had differential thrust working fine using ArduPlane, but trying to over-ride the fail-safe measures to get it into Auto was getting too complicated. I have tried switching over to Rover, but have not figured out how to set up differential thrust.
Next time I am at the lab I will get measurements, but we have 3 motors (middle is just thrust) and it works fine with a good turn radius (may be a shallower hull).
We have a 12v motorcycle battery. For sonar we have an Airmar DT800, being NMEA 0183, it is easy to pull off ASCII depth data (via a separate Raspberry Pi). Here is a short video:
Here is another thread I started on the topic.