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.
I also kept getting this error message.
Eventually I managed to get the ROV in the water and it worked very well.
I did have two problems:
When switched to depth hold mode the ROV shot to the surface or to the tank bed.
In Stabilise mode the ROV held heading very well but rocked.
Otherwise the test was good, no leaks.
I did reset the depth at the water surface.