Uranium recovery from seawater was investigated in simulated seawater in the laboratory and in natural seawater from the coasts of China with different amidoxime-based (AO) ultrahigh molecular weight polyethylene (UHMWPE) fibers. The capacities of adsorbents AO-UHMWPE-1 and -2 were 4.54 and 2.41 mg U/g-adsorbent, respectively, after 24 h of adsorption in the simulated seawater with 330 ppb U. Their capacities were 2.93 and 1.95 mg U/g-adsorbent, respectively, after 42 days of adsorption in simulated seawater flow-through experiments with 3.3 ppb U. However, because of sediment and marine organism contamination, the capacities were 0.25 and 0.04 mg U/g-adsorbent, respectively, after 68 days of adsorption in natural seawater in Xiamen. The capacity of AO-UHMVVPE-7 was 1.41 mg U/g-adsorbent after 15 days of adsorption in natural seawater in Daishan. The average capacity of AO-UHMVVPE-7 was 1.50 mg U/g-adsorbent, which was 18 times greater than that for V after 15 days of adsorption in natural seawater in Daishan. Results indicated that there were many factors affecting the adsorption capacity of uranium. In addition to the character of the adsorbent, including degree of grafting, functional group density, and AO conversion ratio, the marine hydrological conditions, such as temperature, flow velocity, turbidity, etc., are also crucially important for uranium extraction from seawater.