Not sure if that's sarcasm, given the existence (or, not) of practical fusion technology.
We do of course know how to power large ships with nuclear fission. The US Navy has been doing so - for generations(!).
Only issue with that is the US military is a "money is no object" scenario. Price container shipping at the unit cost of hauling it on a naval nuclear reactor, and we'd be looking at degrowth anyway. It's proven, practical technology, but very very expensive.
(Other countries with nuclear deterrents run their sub fleets on fission, but even those with nuclear subs have found it too costly for their surface fleet).
