The one technology that I thought might work really well here is flow batteries. You need a couple of sets of tanks, but then you can do charging by pumping out the old electrolyte and pumping in charged electrolyte. But these seem to have stagnated, possibly because they're dependent on expensive short lived membranes.

I wonder if there's significant scope for offsetting electricity consumption by adding deck renewables. Not for container ships, but maybe for tankers .. which are only needed because of fossil fuel consumption elsewhere. Hmm.

The main obstacle here is the energy density of those is much lower. We're talking 20-30wh/kg. So you would need about 7-8x more mass compared to a 170wh/kg battery. For ships like this (capable of 5000km), the tanks would take up most of the ship with barely any space for useful load.

Otherwise, it's not a bad idea as you could pump the pre-charged fluids in/out and even ship the fluid around with tankers; which opens up the possibility of fueling at sea. Spills would be bad. But probably better than oil as the toxic fluids dilute more easily. But it would still be a bad day for marine life. It might work for shorter ranges.