Photocatalytic hydrogen evolution from natural seawater faces the severe challenges of abundant salts, which adsorb on the active sites and result in undesirable side reactions and photocatalyst poisoning. Herein, a series of main‐chain‐engineered discontinuously conjugated polymer (DCP) photocatalysts is presented with bifunctional crown ether (CE) structures for hydrogen evolution from seawater. The hydrophilic CE can significantly inhibit the aggregation of DCPs induced by salts. Meanwhile, cyclic CE can effectively adsorb cations to uncover the active sites to increase their interaction with protons, which can increase the hydrogen evolution rates and significantly reduce the efficiency roll‐off in natural seawater. Through atomistic studies, the formation of hydrogen bonds with bifunctional CE is elucidated and further analysis of the microscale mechanisms is also conducted using molecular dynamics and ab initio techniques. This work suggests that CE‐based polymer has the potential to enhance its ability to produce hydrogen through photocatalysis using seawater. The first example of incorporating crown ether structure into polymer photocatalysts is demonstrated via a main‐chain‐engineering strategy. The innovative approach significantly reduces ion adsorption on the active sites, resulting in less hydrogen evolution reaction (HER) roll‐off in natural seawater. P‐8CE, in particaular, shows remarkable results with 200% and 258% higher HER than the model photocatalyst, PCzDBTO, in pure water and natural seawater, respectively.