[walrus01]

This will be dependent on big advancements in shore based electrical power costs in the future, but is not totally far fetched:

1) ridiculously low $ per kWh electricity from multi hundred megawatt scale photovoltaic plants may may cracking hydrogen from seawater economical. It takes a ton of power, but if your power is almost free, it can work.

2) store the hydrogen. This has its own technical problems but in terms of energy density can replace heavy fuel oil

3) power ships from massively parallel arrays of hydrogen fuel cells, driving electric thrusters such as current generation large azipods.

Or, if there is some amazing advancement in Wh stored per kilogram/cubic meter of battery, in battery technology generally, skip the hydrogen step and have short to medium range small cargo ships the size of "geared" containerships which recharge in a fashion similarly to the upcoming Tesla semi tractor.