Can humans drink a lot of seawater?

Believe it or not - if you drink only seawater, you will DEHYDRATE. The reason is that the seawater is saltier than your blood so the osmotic pressure pushes the water out of your blood into your digestive tract and kidneys to try to dilute the salt water down to the same saltiness as your blood in order to establish equilibrium.

That's the reason for the famous lines from The Rime of the Ancient Mariner:

Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink.