Why should we not drink seawater?

A long time ago a group of people were shipwrecked, but they had been able to save themselves and now they were floating around on a raft that they had constructed out of wooden boards from the ship that came floating up to the surface. They had no food and no water and they were so far away from the main land that they had no hope of being discovered any time soon.
Because they were very thirsty, life became very hard on the raft after several days. Some of the passengers started drinking seawater in despair. But this made them die of dehydration pretty soon.
How could this have happened while they were drinking water? It happened because seawater contains a lot of salt. When salt enters your body it will absorb a lot of water through a process called osmosis. This will cause the water content of your body to fall, which causes serious dehydration.
That is the main reason why we cannot drink pure seawater and why salt is removed from seawater during drinking water preparation processes.