This provides info from COP28 on an ammonia fueled ship. A demo ship has been built. Article describes some of its characteristics.
Ammonia is easily prepared from green hydrogen in a well known catalytic process. The process was developed by Fritz Haber in Germany pre World War I. He received a Nobel Prize for the discovery. (Engineers help make it practical. Its usually known as the Borne-Haber process.) He also invented use of poison gas in World War I. (And later was outcast in N@zi Germany due to Jewish ancestry.)
Ammonia liquefies under pressure making it easy to ship and handle compared to hydrogen that is either compressed in heavy cylinders or liquefied at very low temperatures. Ammonia vapors are toxic. Hydrogen is extraordinarily flammable. (Very low lower explosive limit in air.)
I read the article to say that ammonia can be used directly in diesel engines as a fuel. Higher consumption than diesel fuel but not a problem with the right equipment. This can produce nitrogen oxides which they scrub with ammonia.
This is a demo ship. Permits not yet issued to operate many places. And while ammonia is widely available, most comes from natural gas. Green ammonia production requires investment.