either sub or aircraft carrier reactors might be somewhere around 50 to 220MWe, panamax might need 60MWe tops, regular land based PWRs are more like 300MWe and up. the smaller you go, the higher enrichment you need, but also military propulsion has different priorities, they use 90%+ enrichment in part because they can, and in part because this gives them massive excess reactivity, which means power level can change ridiculously fast. tradeoff is that spent fuel has much more useful uranium, and it's overall expensive, but you also don't skimp on your doomsday ride so it's all fine. commercial powerplants are physically capable of doing slower load following, but it's more economical most of the time to just use full power in order to best utilize fissile material. what you're proposing would have all disadvantages of both, because no way in hell this thing will run on standard, low enriched fuel for PWRs, it might need something maybe more than 5%, maybe closer to 10%, perhaps more, which means problems, because it means worse proliferation risks than with normal fuel and it's already fuel that goes around, and can be taken over in some unfriendly waters; higher enrichment also means it'll be much more expensive, both because of more SWU needed, but also because it's a specialty product that requires extra licensing; and it also won't be as compact and responsive as military reactor, because civilians don't get to play with HEU like that; and also it will require refueling after some time, maybe longer than regular-sized PWR (refueling every year to three) that probably will require visit to manufacturer to do refueling there, which would be, everything else equal, a bit harder than in regular powerplant because it needs to be done in a drydock

it has all disadvantages of SMRs but also you can steal them on high seas and it's probably great for diplomacy if some random ass pirate get hands on that

iirc us navy loads their reactors with 93% enriched uranium, the same grade that is used in (american) nukes (and also in couple of very special use cases like oak ridge high flux reactor fuel). can't hand this out just like that. one fuel load is expected to last entire ship lifetime. the less enriched grade you use, the bigger reactor becomes and refueling has to be more frequent

The conclusion of the NS Savannah was that it would have been economical after the oil crisis of the 1970s caused a price spike in fuel costs. Ports also need facilities and training to handle nuclear fuel. Once you have that, it's perfectly viable.

Unlike energy generation on land, there isn't a lot of alternatives for decarbonizing marine transport.