US nuclear submarines consume highly-enriched uranium, that's nearly as (and sometimes more) pure as the weapons-grade version. That doesn't mean oceanic reactors aren't possible, it just means that military subs are a bad example.
Nuclear powered non military ships do exist, it just not economically feasible .
It is hard to compute the economics of small nuclear reactors that use highly enriched fuel. A lot of it is funded by defense needs.
Mixed use is largely to keep defense manufacturing active not because they are economically effective.
If nuclear civilian ships were cheaper, there would be efforts to make a lot of them (In Russia and China if not other countries etc)