As Eric has mentioned the ROV should be powered with +12V because any higher voltage could result in damage to the ESC's. Because the ROV Powers up with the tether connected and you have video in the cockpit tells me that the tether is working properly. Not seeing any values in the cockpit telemetry data points to a possible issue with the Arduino code not being loaded properly.
1. try loading the Arduino code again making sure there is no errors.
2. there is a keyboard icon on the top of the cockpit that you click on to show you all the keyboard controls. One of these turns off the ESC's so if you hit that key accidentally then nothing will work until you enable it again.
3. When you said the motors work what do you mean by that. What do you do to make them run or do they just spin on there own when you power up the ROV. If this is the case then make sure they are programmed correctly. Then they need to be calibrated in the cockpit which is where you are probably running into issues because nothing is working.
You could also burn a new image onto an SD card and try running on that and upload the Ardunio code. Until you get the image and Ardunio code in sync on the same version you may run into issue like the ones you are experiencing.