Ammonia, hydrogen, and methanol have emerged as promising zero-carbon or alternative fuels for maritime applications. Ammonia shows considerable potential in fuel cells and internal combustion engines, though efficiency and emission performance improvements are still necessary. Methanol, a well-established alternative, offers benefits as a hydrogen carrier, but green production methods need further development. Hydrogen faces significant storage and transport challenges, particularly in its compressed and liquefied forms. As the maritime industry pursues decarbonization, it must carefully weigh the advantages and limitations of these diverse fuel options to make informed decisions for a sustainable future. This chapter summarizes the unique benefits and challenges of ammonia, methanol, gaseous hydrogen, and liquid hydrogen in marine fuels. It highlights the importance of overcoming these challenges through continued research, development, and industry collaboration to achieve a more sustainable and environmentally friendly shipping future.