The answer is quite simple: Seawater contains too much salt for human consumption. Although we can consume salt in small amounts, our bodies cannot handle taking in as much salt as is contained in seawater. Drinking seawater would have horrible consequences, including severe dehydration and even death. Our bodies need a certain amount of salt to function. We take in salt but we also drink much through the day which dilutes the salt. Without this dilution, dehydration is inevitable.
When you drink liquids, your body rids itself of anything it doesn’t need by making you urinate. If you drink seawater, the same would occur, only the salt would remain. This would cause severe dehydration, and even possibly death. Your kidneys would try to process the salt water but would struggle doing so.