@SDI, indeed, the website moved last month. This is the updated link: https://www.ardusub.com/troubleshooting/troubleshooting.html. However, I don’t think your problem is addressed there.
First check that the Pixhawk is mounted flat in the electronics enclosure, and that it has not come loose. Then perform the accelerometer calibration, and at each step, set the vehicle on the ground lying on its left side, right side, and back, and nose. This cant really be done during ‘nose up step’, but do your best to get it straight up and down there. The key during each step is to have the vehicle sitting completely still before hitting the ‘Next’ button.
After you are finished with the calibration, set the vehicle right side up and look at the horizon indicator to see if it is level. Reboot the ROV and note any change in the horizon indicator when it connects again. If it still is not looking good, you can go to the sensors menu where you do the calibration and click the ‘level horizon’ button, which should bring the horizon level.
Please let me know your observations.