You need to consider that the concentration of NaCl (salt) in seawater is around 3.5%, while in our blood is around 0.9%.
When you drink seawater, then, you have a lot more salt in your body, which cannot pass inside a cell, but (in order to keep the balance between the inside and the outside of a cell) recalls the water outside.
Kidneys start to work at full load to expel salt into the urine, which unfortunately can be much less salty than seawater, therefore we "waste" more water into the urine in order to expel the salt than the water we drink.
This leads to dehydration and starts a vicious cycle that can bring to death.