Water from the sea is too salty to drink. Although your body needs salt, it is possible to have too much of it. See, your body is made of cells which contain lots of things, including water, salt, and other chemicals. The amount of chemicals that are dissolved in the water inside your cells keep a balance with the amount outside the cells.
Throughout the day, you drink water and eat salt (in different foods) and your body manages to keep this balance pretty well. But if you suddenly take in way more salt than your body can handle without enough plain water to dilute it, that balance is hard to keep. The salt water enters your bloodstream. Then your blood cells sense that the balance is wrong, so they send out their water to dilute it. They may also send out some of their important chemicals to balance out the chemical charge of the salt ions (sodium++ for instance). Well, this is just too much – the cells get dehydrated. They need water on the inside, but they are forced to send it out. This is very bad for your blood. It may even raise your blood pressure, which can be very dangerous.
Also, in your intestine, the cells may do the same type of thing by secreting a lot of their water to maintain the balance of how much salt is dissolved in water. The extra water going into your intestines and colon will give you diarrhea. Overall, your entire body would be dehydrated, and this can do a lot of damage to your organs.