Although it provides many major necessities for us, seawater can't provide drinking water. Due to it's high concentrate of salt, the water, if consumed, would actually dehydrate us and make us even more thirsty. However, many of the sea mammals have adapted to life in the ocean. Animals such as whales and dolphins don't drink the seawater, but rather get the water they need from the food they eat.
The ocean is the ultimate provider of Earth. Not only does it provide a home for marine animals, the ocean's plant life provides 70% of the Earth's oxygen. Its surplus of food makes it possible for both carnivores and herbivores to live off of it. Industry thrives off of the ocean from fishing to global transports. If it weren't for the ocean, the vast majority of it's creatures would die out. It is important that we conserve it.
Much of our global climate is affected by the currents of the ocean. They are the reason why many places are warm and cold in terms of their location. Ocean currents transport warm water and precipitation from the equator towards the poles and with cold water the exact opposite.
We have little knowledge of the ocean as a whole, including what is in the deep sea. Many deep-sea creatures exist in the lower layers of the ocean and have adapted to the darkness and the high pressure. Some species we know of, but most are yet to be discovered.
As you get lower in the ocean, temperature drops and pressure rises. Density and salinity are affected too. If in cold water, the bottom may not be as dense and salty as the top.