Cargo ships burn incredibly dirty fuel, the working conditions are bordering on modern slavery, and it isn't sure if some cargo travel routes will stay viable in the near future - the Panama Canal has issues fitting the latest and greatest ("Postpanamax") ships and it's dealing with severe water shortages, and the Suez Canal is under threat from Iran as well as from a repeat of the infamous Ever Given incident.
There are a lot of people working on converting cargo ships to ammonia fuel. This will at least burn clean producing only nitrogen and water with appropriate engine tech. The ammonia can come from green hydrogen, so it could be 100% clean tech and ammonia doesn't have the problem of needing to be compressed and has good energy density.[1]
The problem is, shipping is incredibly price sensitive (as evidenced by the industry's SOP of using cheap flag states, exploiting labor and burning bunker fuel aka refinery residue). As long as said refinery residue is cheap, they'll keep the old guzzlers for as long as they can, and only convert a couple "flagships" to ammonia to appease ESG investors.
It just doesn't scale, there's only a few passenger seats available, and if we used the whole cargo ship to transport people it wouldn't be low carbon anymore.