Fusion, at least of the most commonly pursued DT variety, is terrible for mobile applications since its power density is so low. The ARC reactor concept (190MW(e)) weighs as much as several WW2 destroyers.
I'm going to go out on a limb and guess they modern fission plants that are not designed with portability in mind also have really low power densities. Just imagine the weight of the cooling towers. And yet, very different designs with different requirements can be made to fit in a submarine.
I'm not saying it's going to be possible to run container ships on fusion, just that using a fixed research reactor as a data point probably isn't very useful.
Actually, no, fission reactors have much higher volumetric power density. This is inherent in the technology -- in a fission reactor, coolant flows through the core, with large surface area for heat to transfer from the thin fuel elements. In a DT fusion reactor, the coolant has to flow in a blanket around the core, and all the power has to radiate through the surface of the reactor itself. The square-cube law comes into play.
