The total known ground resources are estimated to be roughly seven times larger than the current mining reserves. So this gives you at most a factor of ten. Of course there are other unrealistic assumptions in those estimates. Realistically you would use other sources of electric energy for instance. I don't now if the Thorium reserves are roughly four times larger than the current Uranium reserves or four times larger than the amount of Uranium that could feasibly be extracted from the ground. In any case, both are only a solution for the near future, that is the next 50-100 years or so.
With current technology it would not make sense to extract Uranium from the Ocean, its concentration is $10^{-9}$, it can be commercially extracted from rocks with $10^{-4}$ concentration. It does not possible to filter huge amounts of seawater for such insignificant quantities of Uranium.
