Hagard, I'm super excited to see you trying to control the ROV using an Arduino. I also have been getting parts together to build an ROV and want to use the Arduino to control the vehicle. I'm interested to see how your code works!
My approach was going to be a little different, I had planned on using the Arduino IDE to upload the code to the unit, but only using one unit topside (Arduino Mega) managing the relays and using input. I like the idea of having the nano subsurface collecting all the data from the sensors.
Have you looked into Processing?
This is another IDE that integrated very nicely with the Arduino and my plan is to program the physical interface here. I think it may be possible to stream video into the program using an EasyCap video to USB converter although this has been untested and I'm unsure of how it will work.
Anyways, let me know how your code works out, very interested to see!