Hi @KaiBo -
As with the discussion earlier in the thread, the approach you describe isn’t particularly logical! If you need information on the vehicle orientation, reading a dedicated IMU with your Arduino Mega and controlling thrusters is certainly possible, but would require developing your own autopilot code to accomplish this - not to mention that messages from a DVL would need to be translated from TCP/IP, or taken in via serial and fused with your motion sensor data.
Your approach would seem to be comparable to taking a car with functional self driving functionality, and rigging up some additional external system that pushes the pedals and turns the wheel based on what is shown on the cars internal display - a bit overcomplicated of an approach when compared to working with the autopilot software directly.
The ArduSub autopilot can read the data from the DVL, and use this to accomplish auto routes navigation via thruster actuation already! If you’d like to add your own code to control this autopilot behavior on a higher level, I’d recommend checking out Lua scripting functionality, as discussed here and documented here.