There's multiple orders of magnitude in difference between the amount of fuel a ship carries, and the amount of oil a tanker can hold. Think measured in hundreds of thousands versus tens of millions of gallons.
If we start accounting for every single externality for every solution for a problem we come up with, we might as well stop leaving the house at all.
The Fremantle Highway was burning and at risk of sinking in the Wadden Sea, off the Dutch coast of just a week ago. This would have severely damaged the entire habitat - and that was a car carrier only. Spills from minor incidents are a concern as well, especially in habitats that are important, slow to recover and already under a lot of pressure.
A fuel oil spill from a cargo ship is not a trivial externality to be ignored! Yes, it's a smaller order of magnitude than an oil tanker. But it's not de minimis.
