water

The scientific explanation for humans not being able to drink seawater is this:

The amount of sodium chloride, or salt, in human blood and urine needs to be constant.

Drinking seawater temporarily increases the concentration of salt in the blood, so the only way to excrete through urine is by sacrificing internal water from cells to dilute salt.

The cells eventually give so much water that they die from dehydration, quickly followed by organs and the organism.