It will do it, but it’s by no means a simple task. The only reason I’m suggesting it’s possible is because you’re not going too deep. The longer the cable run the more difficult it becomes. Is changing multiple sets of batteries an option for you? It’s probably the easier option.
The ROV uses between 0.5A and 0.8A just to run the computers, With each motor using up to 10A. So you’re probably looking at least a 20-30A 12V DC supply. A linear supply with voltage tracking would be a lot better than a switchmode, as the noise from a high current switchmode could interfere with the ROV or the network.
You will need a big chunky cable to handle the current without getting a massive voltage drop too. About 8-10 AWG would probably be okay. But this will also introduce drag on your ROV and make it much less maneuverable.
Are you planning to run the ROV just from the DC supply? or run it with the batteries on board as well and use a charge controller? Have you used the IMU wires or spare wires on your 2.7? You will need to use more than 1 pair of the wires potted thru the end-cap to handle the current. I would suggest using all the spare wires you have available.