It's a matter of maintaining the appropriate balance.

The human body works best when there is ~9g of salt/liter (based on normal saline used in medicine) which is necessary for electrical signals and chemical reactions that the body needs to keep functioning.

Seawater, by contrast, contains ~35g of salt/liter, more than tripple the concentration of the human body.

This means that drinking seawater intoduces far too much salt, but drinking water with no salt at all can dilute the salt concentration of the body, as some salt is lost via urine and sweat. Either of these extremes can cause problems.