Working in deep water presents its own set of challenges.
For long tether runs, use negative tether because it can transmit more power and has minimal drag. A short section of performance or neutral tether should still be used at the ROV unless the tether can always be held above the ROV.
Add a weight to the tether several meters behind the ROV. Make sure to distribute the stress of the weight connection along a section of tether rather than tying the weight to one point. Remember, you will need to retrieve the weight, and this can be difficult if the tether is long.
To speed descent, grasp a disposable weight in the manipulator. When you reach your operating depth release the weight in order to be able to pilot the ROV better.
Use the vertical thruster to pitch the nose downward, and then use the horizontal thrusters to add more power to dive or surface.