* There are 6.3 million tonnes of thorium in reasonably assured deposits (World Nuclear Thorium)

* Uranium exists in seawater at an average concentration of 0.003 ppm (also World Nuclear Uranium)

* There are about 332 million cubic miles of water on Earth, 96.5% of it is in the ocean (USGS). At a density of 1 gram/cm3, this comes out to 1.4 yottagrams of water, or 1.4e21 kg)

* At 0.003 ppm, this means there are about 4000 million tonnes of uranium in seawater

There is effectively zero thorium in seawater. Seawater uranium extraction with breeder reactors is likely to be more economical than harvesting low-grade thorium from the crust.