It needs to be cost effective (which might include profotabilit6), be feasible to implement, sustainable, and a bunch of other stuff too.
Aircraft carriers and subs being the only transportationethod using nuclear is a good sign that it isn't practical for shipping. That is likely a combination of nuclear requiring highly trained staff that are not just out in the work force, the ability to procure a reactor and maintenance parts, the ability to obtain fissible material, the ability to dry dock a ship with a nuclear reactor, and a bunch of other stuff that could even be affordable without being practical for reasons beyond costs.