Signal and power cable over long distances
Real ROVs often run anything up to 1,000V or so at the head end of the cable (Nothing like that when it gets to the vehicle under load of course), sending significant power down to the vehicle is problematic. 

The other gotcha is that you really want the cable to be neutral buoyancy overall at the vehicle, and that is a lot harder then it sounds if you have lots of heavy copper in there.

If I was doing it I would put the batteries (Lead acid is good) on the vehicle, not at the surface (No need for Lithium, that is weight efficient, but you need ballast anyway to make the thing submerge), and would look seriously at fibre for the data link (Small switch with an SFP at each end, 300M is easy, and a 300M drum of single pair glass is not exactly expensive in the scheme of underwater electronics, a very quick search implies ~£150 which is not bad for a 300M gigabit speed link).

Now unless you have played with underwater doings a LOT you will discover very quickly that keeping water out is HARD, forget 300M, just doing it at snorkel depths will give you fits, I forsee lots of PUR resins and weird issues with osmosis in your future.