The current vehicle is wired to use two parallel packs of 3 Lithium batteries in Series- either LiFePO4 (nominally 3.2V each or 9.6V total) or LiMn or LiCo (nominally 3.7V each or 11.1V total). You can power the vehicle with anything between, say, maybe 8V or 14V and it will work. The biggest issue is providing sufficient current- during a full-power drive the ROV needs maybe 15 Amps or so of current. Alkaline "C" batteries and most people's bench-top power supplies can't provide that amount of current, no matter what the voltage is.
The very earliest OpenROV designs used "C" batteries. The two battery packs were wired in series, with 4 batteries in each tube, to provide 12V. This design sort-of worked, as long as you moved very slowly, and for only a short period of time. The rechargeable lithium batteries are considerably better.