A 12V power supply can be used as an alternative to using the batteries for testing the ROV. If you have not epoxied the aft battery tube end caps in place yet you can just connect the power supply to the battery terminals, otherwise you would have to make up a battery adaptor to slide into the battery tube to make contact. Looking at the schematic, the other option would be to connect the power supply to the AUX power input (J11), but keep in mind that this input has no diode protection and accidentally reversing the wiring would likely fry your ROV. The other thing to note is that the AUX power input bypasses the battery current monitoring circuit so you would get invalid readings for these.
There is one other point to keep in mind when using a power supply to power the ROV, the inrush current on power-up will cause a brown-out on the beaglebone at startup if the power supply is under-rated. Using a 6A benchtop power supply, the only way I was able to get the beaglebone to startup normally was to switch off all ESCs prior to switching on the ROV and then turn on the ESCs one at a time (the ESCs have quite a large inrush current). Using a 3A supply I had troubles even with the ESCs switched off and would find that the webcam would fail to load successfully.