At long last I have been able to test my BROV in a controlled environment. My local university kindly allowed me to use there ocean test tank whilst they were preparing for another project.
Initially I had some problems with my Xbox controller that would registered with QGC but would not control anything.
Cool, that looks like an awesome tank to do your testing in!
I believe that your vertical thruster directions are flipped, which explains the ROV thrusting in the wrong direction when holding depth and amplifying rather than damping rocking. If you go to the parameters tab of QGC and reverse the directions of motors 5 and 6, it should fix the problem. Does this sound right @jwalser?
I will give that a whirl, if you will excuse the pun. The thought did cross my mind but I dismissed it pretty quickly as I thought it is too easy, doh!