I assume if you can extract Uranium from sea water, you are very close to extract Thorium. In any case, we are all set up to use Uranium and Thorium still require a lot of R&D, politics and safety regulation before we can start generating commercial power with it.
Thorium isn't soluble in water. (But there's enough thorium on land to last us for millennia, if we ran all of civilization on it at our present usage rate.)
But I agree - for sea water extraction to be practical, we would need to more or less exhaust the cheaper supplies. I suspect that, even if our consumption continues to grow at the present curve, if we don't figure out other ways to power civilization by the time we exhaust Uranium and Thorium, we really deserve to run our of fuel...
