Would it be ok to use 14.4V to power the ROV instead of 12V? I ask because there are many affordable 14.4 cordless drills out there. Would be nice just to use one of their battery packs and chargers. Sure, it's not as efficient as using 12V directly, but for simplicity of using straight off the shelf components would be nice.
Seems like cordless drill battery packs would also supply enough amperage, I hope :) Also, could run them in parallel if needed.