https://en.wikipedia.org/wiki/Triple_E-class_container_ship

It seems like ~ 1 gallon of fuel per day per container

shipping across the pacific is 15-30 days, from asia to europe is 30-45 days.

seems pretty efficient, per container.

Probably dwarfed by the costs of getting the container to and from the ship.

Even if you compare fuel usage against inland barges, which capacity-wise are much closer, they use something on the order of magnitude of 25 metric tons of fuel every 2 to 3 weeks or so, depending on cruising speed and number of stops of course. Assuming about a 208 TEU capacity, and 1.8 tons of fuel a day, that'd be roughly 2 or 3 gallons of fuel per container per day.

People tend to really underestimate how ridiculously efficient ships are per unit of cargo shipped.

Realistically, if you want to make a difference in emissions right now, figure out a way to move inland shipping (the port-to-actual-destination leg) from trucks onto trains and barges, only doing last-mile trucking. I don't think the barrier to that happening is actually technical, nor is it financial because those modes tend to be cheaper. The economics around these modes of transport are well established. It's primarily that they are slower, so you tie up a lot of cashflow into goods in-transit.

Because the cost of shipping is so low relative to the cost of the goods itself, it typically does not make sense to save pennies on the transport while tying up serious cashflow for longer. I'm honestly surprised there is no financial institution that has made a financial product around financing the cashflow gap created by greener modes of transport: the pay-off is predictable, and the risks are low.