One cable per car to its charging port - power can be transmitted in that direction too.
Edit: Quick reality check: About 500 MWh from the batteries. An oil-based ocean crossing of a similar sized vessel uses 280 tons oil, corresponding to about 3 GWh.
So, meh. Probably not worth all the hassle - the cabling, recharging the cars after the crossing, dealing with various extra fire risks, etc.
Edit: It will also become more feasible with energy density increases in batteries. I hope it's not entirely unrealistic to expect a doubling in the next 20 years.
I was first hoping the batteries could be used to reduce the extra crossing time (12 days vs 7 days for oil) significantly, but I'm guessing it would only reduce it by 0.5-1 day or so.
[1] https://www.nuclear-power.net/nuclear-engineering/thermodyna...