Why can’t people drink seawater?

No matter how thirsty we become, we cannot drink seawater. Seawater contains an electrolyte several times thicker than body fluid.
Therefore, you will feel even thirstier after you drink seawater as body fluid electrolytes will also become imbalanced and your body fluid will thin out. As a result you won’t feel thirsty anymore but your body is actually dehydrating. This is called Voluntary Dehydration.