Our bodies are mostly water (~75%). The water in our bodies supports many functions, including waste removal and temperature regulation. Since water is consumed and expelled by these functions, we need to constantly replenish our water reserves to survive. In normal circumstances, the saltiness of water in your cells is the same as the saltiness outside your cells. But when your cells consume water, the solution becomes more concentrated, and the natural process of osmosis allows water from outside your cells to pass into your cells to achieve equilibrium. That’s how we absorb water as per need.
The opposite happens when drinking sea (salt) water. The salinity outside your cells increases rapidly and water from cell moves out, causing the cells to dehydrate. A person may die of extreme dehydration.