“Around 90% of all goods traded globally are transported by sea...Marine transport produces around 2% of global greenhouse gas emissions.”
Sounds like a bargain to me, frankly.
It might be simpler to require cleaner marine fuel, but marine fuel is currently a polluter at a large multiple of its share of greenhouse gas emissions.
Also, ammonia is a very good candidate for an economically viable energy storage and transport medium for renewable energy generation. Hydrogen is expensive to store and embrittles metal containers and pipelines.