It would still require a lot of power and a lot of infrastructure to do it. If a country or organisation would try to go down that route others would see that t

It would still require a lot of power and a lot of infrastructure to do it. If a country or organisation would try to go down that route others would see that they spend a lot of effort on slurping seawater through machines they can’t explain with anything else. That is the bottleneck how nuclear non-proliferation is enforced. You can hide your intentions, but you can’t hide that a lot or energy is flowing into a process you don’t have a good explanation for. Because of this detecting a sea water sourced extraction would be even easier than a nuclear program based on mined uranium. (Because you need to enrich from an even weaker starting condition.) What happens after detection is either diplomacy (they ask the country to stop nicely, reminding them of the consequences) or force (bombing the facilities).