Even though 71 percent of the earths surface is covered by water only about 2.5 percent of this water is usable. Much of this water is locked in glaciers and ice caps. Drinking sea water may not seems so bad but with all of the salt in the sea water your body can not get rid all of this salt fast enough so you will get even more dehydrated. If you drink a large amount of sea water your body could fail and you could die.
As the oceans depth decreases the pressure rises and the the temperature decreases. Near the bottom of the Twilight Zone the tempature starts droping dramaticaly due to the decreasing sunlight, see graph below.
The ocean provides food and it is mined for salt, sand, gravel, and some manganese, copper, nickel, iron, and cobalt these can be found in the deep sea. Also the ocean is drilled for crude oil. The ocean plays a critical role in removing carbon from the atmosphere and providing oxygen. Also the ocean support so much life that we have not even discovered yet. Ocean is a vary vital resource and we should do all we can to protect it.