I'm skeptical that onloading and offloading containers of batteries is something that can be done quickly and easily on average. Suppose you're using batteries (probably lithium iron phosphate if it's based on current tech) that can charge at 1C. That means you can fill all the batteries completely in an hour just by charging them from external power (assuming enough current is available). I don't know how long loading an unloading take, but I assume most ports would have lines of ships waiting to use the cranes, and if you can't get everything done in under an hour you're better off just charging them on the ship.
Suppose you're a coastal city that wants to setup a charging facility for passing local ships. You can either build a full seaport (which might not be practical depending on geography) with a crane and battery storage and charging facilities, which can handle ships one at a time, or maybe two or three if you have the dock space. Or you can run a high voltage line out to a buoy, and ships can hold position out in the water and connect to the power receptacle at the buoy for an hour and charge. If you want to have ten ships be able to charge at once, you run ten high voltage lines out to ten different buoys. I would suggest that the latter option is probably cheaper and easier.
It's also not out of the question to have HVDC lines that connect continents to each other for power sharing (i.e. buying and selling solar power based on where the sun is shining), and if such cables exist across oceans it's not much of a stretch to imagine a string of charging stations at regular intervals for passing ships to use, even across the Pacific.
Even better would be some system where you can have a continuous connection to power, like the pantographs connecting to overhead lines in electric street car systems, but I have no idea how you would do that at sea.
If you containerize batteries (which is a thing with some electric ships already), swapping the containers is a few minutes work. Most container harbors routinely process container ships with thousands of containers using automated cranes, autonomous ground transport, etc. I don't think that's a process that takes a lot of time. They probably measure that in numbers of containers per minute. So, I think that we can establish that swapping a few batteries should not be that big of a deal technically.
Recharging those containerized batteries would require some infrastructure. Conveniently, off shore wind parks are currently pretty popular, can generate lots of GW of power, and can be close to shipping routes. There are all sorts of solutions that you could imagine here. You could even shuttle batteries to and from ships that need them so those ships don't even have to make an expensive detour.
That work also would include high current high voltage connections. Which are suitable for use at sea... Which makes me thing that it is not exactly as fast process.
Plus you likely want to stack these to one part of ship and then risk having them relatively close each other which in case of fire could be risky.
What if this assumption isn't true: dispatchable energy and opportunistic energy are two very different things.
If you want to build out battery storage to get a stable grid from intermittent sources, chances are you'd end up with cargo-container sized standard modules anyways, instead of every port having its own bespoke facilities reinvented in some folksy local style
Power delivery could be an issue. I guess it depends on how many ships a day you're serving. If there's just one or two a day, your infrastructure needs to handle short bursts at high capacity but then sits idle 90+ % of the time. If you're serving a hundred ships a day, it sort of just averages out and you can be at close to maximum capacity most of the time. I'd imagine most power utilities would love to have customers that consistent.
Delivering power to the ships would be interesting. I suppose it would have to come in at high voltage (either AC or DC) and be stepped down to a suitable pack voltage. That means each ship would have the equivalent of an electrical substation on board.
Because in the time it takes to physically swap some batteries and charge others, you could have just charged them all and that would be simpler and easier.
One of the things that's counterintuitive about batteries is that if a battery is twice as big it still takes the same time to charge, not twice as long. (That's assuming you can supply enough power to charge at the fastest rate the batteries can handle.)
Suppose you're a coastal city that wants to setup a charging facility for passing local ships. You can either build a full seaport (which might not be practical depending on geography) with a crane and battery storage and charging facilities, which can handle ships one at a time, or maybe two or three if you have the dock space. Or you can run a high voltage line out to a buoy, and ships can hold position out in the water and connect to the power receptacle at the buoy for an hour and charge. If you want to have ten ships be able to charge at once, you run ten high voltage lines out to ten different buoys. I would suggest that the latter option is probably cheaper and easier.
It's also not out of the question to have HVDC lines that connect continents to each other for power sharing (i.e. buying and selling solar power based on where the sun is shining), and if such cables exist across oceans it's not much of a stretch to imagine a string of charging stations at regular intervals for passing ships to use, even across the Pacific.
Even better would be some system where you can have a continuous connection to power, like the pantographs connecting to overhead lines in electric street car systems, but I have no idea how you would do that at sea.