I read this story today which outlines some of the issues:

https://insideclimatenews.org/news/28042021/shipping-looks-to-hydrogen-as-it-seeks-to-ditch-bunker-fuel/

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

A carbon tax would greatly accelerate the development of green technologies for shipping because it would make them more cost competitive with oil, but the tax needs to be high enough.

We have a new blog post outlining some of the challenges and opportunities:

https://greenexecutive.com/front-page/blog-categories/climate-change/how-decarbonizing-shipping-could-unlock-a-global-energy-transition

I just read this story about the use of molten salt nuclear reactors for commercial shipping:

https://www.maritime-executive.com/editorials/modular-molten-salt-nuclear-power-for-maritime-propulsion

Many thanks to everyone who replied! The information is helpful.

Today, I found another good story summarising progress towards decarbonising the shipping industry:

https://www.greenbiz.com/article/zero-carbon-fuels-and-marine-shipping-both-will-and-way

Shell has produced a report about decarbonising the shipping industry:

https://www.shell.com/energy-and-innovation/the-energy-future/decarbonising-shipping.html

I have not read it yet, so I can not comment on its content or credibility.

Here is a report titled "Decarbonising European Shipping - Technological, operational and legislative roadmap" by the European Federation for Transport and Environment (2021):

https://www.transportenvironment.org/sites/te/files/publications/202104_Shipping_Technological_Roadmap_to_Decarbonization.pdf