It’s not a battery problem but it might very well be a power supply problem. I’d check your power supply and make sure the voltage isn’t sagging as the ROV boots up. The ESCs need a fair amount of current when they switch on, your supply should be at least 3A or so for that to work.
If you just want to test the ROV electronics, don’t want to run off of batteries, and you don’t have a high-current bench power supply, you can try jumpering across J17, the pads marked “ESC power switch”. This will force the ESCs to power-up at the same time as the rest of the board does, so the ATmega microcontroller and the BBB will boot after the inrush surge from the ESCs.
You can download the schematic for the 2.8 controller board off of our Github site, if you’re curious as to what that jumper is doing.