[pjc50]

Requires a huge capital investment in batteries, which are then idle for the voyage in both directions.

However, we can think this through a bit: what is a battery? It's a pair of redox reactions, involving two electrodes and either one or two electrolytes. The electrolytes are usually liquids. That led to the concept of the "flow battery": https://en.wikipedia.org/wiki/Flow_battery

So, if the electrolytes can be stored in tanks separate from the electrodes, theoretically you could put the tanks on ships and cycle the electrolytes between countries.