A panamax cargo ship burns ~250 tons of bunker fuel a day and carries ~15k containers. Cruising speed is a little over 1000 km / day.
The energy density of bunker fuel is approx 12500 wh / kg. Lead-acid batteries hold 50 wh / kg while high-end li-ion cells hold 500 wh / kg.
By those numbers it sounds like you'd need approx 6000 tons of high-end lithium cells per day of operation - you can put 40 tons in a shipping container (much more and many ports will refuse to unload), so 150 containers of batteries per day of travel. A 5000km trip is going to use 5% of your cargo space for fuel (where previously it would be closer to 0.1%). That adds up to real money (you could have moved 750 containers at approx $5000 each, so you lost ~3.75 million, but the fuel would have only cost approx 0.3 to 1.5 million USD depending on where you fill up).
For short trips (sub 1000km) I suspect they'd probably use cheaper, less-energy-dense ni-mh batteries as they are safer and cost less than half as much for a comparable charge.