I am not an expert, but I have done some reading on this topic. My understanding is that no/low carbon propulsion for ships is still in its infancy. They are experimenting with a lot of different technologies including electric, wind, hydrogen, solar, biofuels, and ocean currents. Sometimes in combination. Many seem to believe that green ammonia will be the best option for long distance shipping.
You might be interested in this story
Why the Shipping Industry Is Betting Big on Ammonia:
There’s a lot to like about ammonia. This colorless fuel emits no carbon dioxide when burned. It’s abundant and common, and it can be made using renewable electricity, water, and air. Both fuel cells and internal combustion engines can use it. Unlike hydrogen, it doesn’t have to be stored in high-pressure tanks or cryogenic dewars. And it has 10 times the energy density of a lithium-ion battery.
For all these reasons, ammonia (NH3) is gaining favor in the global shipping industry, a multitrillion-dollar machine in need of cleaner fuels to power the freighters and tankers that haul manufactured goods and bulk materials across the ocean. Shipping companies seek climate-friendlier alternatives to petroleum that can propel their behemoth vessels for days or weeks at sea and still leave room on board for cargo.
https://spectrum.ieee.org/transportation/marine/why-the-shipping-industry-is-betting-big-on-ammonia