Start with Basic troubleshooting.
1.start with just the BBB only. Power it up using a usb power adapter from a cell phone or other device. Connect a lan patch cable from it to your computer. Set the IP address on your computer ipv4 to192.168.254.2 subnet mask 255.255.255.0
you should be able to use putty to SSH connect to the BBB using 192.168.254.1You should also be able to use google chrome and see the cockpit browsing to 192.168.254.1:8080
the power adapter may not have enough power to power up a web cam so you can test this in step 2.
if this works then.
2. Mount the BBB on to the control board. install jumpers on J12 to manually power up the Controller Card and BBB using your ROV Batteries. Connect your Lan patch cable from the BBB to your Computer again then power up.
You should still be able to connect to the BBB using Putty SSH and login.
If your web cam is plugged in you should be able to open Google chrome and connect to 192.168.254.1:8080 and see it in the cockpit.
You should now be able to update the Arduino Code from the cockpit.
If this works move on
3. Install jumper J17 on the control board.
Assuming that you have manually programed the ESC's
you should now be able to Calibrate them one at a time. Switch all esc's to off except the one you wish to Calibrate, once calibrated turn it off and move on to the next one. once finished then you should be able to turn them all on and control the motors.
4. If all is working then you can test the Home plug adapter. Install the Home Plug Adapter onto the Control Board and connect it to the BBB. connect up your tether and connect up your computer . All the led's on the Home Plug Adapter should light up and you should be able to connect to the cockpit. At this point you still have the jumpers installed on the control board to power it up rather than using the power through the tether to do it.
If you can't connect to the bbb using putty or Google Chrome then you may have to pair up the Home Plug adapters or you may have a bad adapter
try some of these steps and let us know how far you get.