Uranium makes up a teeny percentage of the particles in the ocean, about 3 parts per billion or roughly the equivalent of a grain of salt dissolved in a liter of water.
The thing about the ocean is there's a whole lot of it, about 1.3 sextilion liters, or 13 with twenty zeroes after it. That's a whole lot of uranium just drifting around when it could be powering your fridge, so since the 90s, Japan has been developing methods to pull it out of the water, with the United States and China following suit.
Uranium exists in water bonded to two oxygen atoms in a molecule called uranyl. Uranyl has a positive charge, so previous methods of extraction took advantage of this.
To capture the uranyl, long strands of plastic coated in a chemical with a negative charge called amidoxime were left underwater for a month, where they would passively soak up that uranium goodness. Then the strands would be brought to the surface, where an acid bath would free the uranyl and regenerate the fibers so they could be put back in the ocean and used again. It's literally rinse and repeat.
IEEE Spectrum: Nuclear Fuel From the Sea