Pumped storage can be constructed out of concrete. It's not necessary for there to be a preexisting basin. This isn't often done because there's not much demand for it and it's very expensive compared to fossil fuels (which pretty much everything is), but it is entirely industrially feasible and is still generally cheaper than any other kind of battery. I realize you are arguing that extracting uranium from seawater is in the same category, but it's not: there are no industrial scale mining operations extracting uranium from seawater, and there is no evidence that existing prototypes would scale up to global power requirements (in addition to the other issues mentioned by the neighboring comment).