(i) All of the organisms in the sea are adapted to salt water. If they were suddenly immersed in fresh water, too much water would diffuse into their bodies and they would die.
(ii) The global ecosystem would collapse, and the oceans and atmosphere would eventually revert to their composition before the evolution of photosynthesis: Almost no oxygen and lots of carbon dioxide.
(iii) The ocean currents will be completely disturbed. Hence it will trigger profound climatic changes at the global scale.

Though 71% of the Earth is covered by water, 97.5% of the Earth's water is in the oceans in the form of saltwater that cannot be used. About 2% of the water is found in the form of glaciers and ice sheets. Thus, only 0.5% of Earth's water is available for human use. It makes fresh water a critical resource.

(i) The movement of oceanic water on the surface and at the depths in a definite direction is called ocean current.
(ii) Ocean currents are in clockwise motion in the northern hemisphere and in the anti-clockwise motion in the southern hemisphere.
(iii) The factors that generate ocean currents are: Earth's rotation, Prevailing winds and Differences in temperature and salinity of ocean water.
(iv) On the basis of temperature, ocean currents are classified as warm currents and cold currents.
(v) The movement of ocean currents from the low latitudes (tropical zones) towards high latitudes (temperate and polar zones) is called warm current. Example Gulf Stream in Atlantic Ocean, North Equatorial Current in Pacific Ocean.

(A) Continental Shelf
(i) A shallow and gently sloping platform extending out from the adjoining continental land mass into the sea is called Continental Shelf.
(ii) It is almost a uniform zone of sea bed with a gentle gradient.
(iii) It is significant in promoting richest fishing grounds.
(iv) Since the continental shelf in shallower, thus enables sunlight to penetrate through the water. This encourages abundant growth of grass, sea weeds and plankton to feed fishes.
(v) The continental shelves have extensive deposits of minerals and mineral fuels.
(vi) Hence, this zone becomes accessible for oil drilling and mining activities.
(B) Continental Slope
(i) A steep slope which descends from the edge of the continental shelf to the deep ocean-bed is called continental slope.
(ii) It forms a boundary between the Continental Crust and the oceanic crust.
(iii) This zone is free from deposits as they are steep.
(iv) The most important characteristic of continental slope is the presence of deep canyons and trenches.
(v) Due to the low penetration of sunlight, the slope has nearly freezing temperature.
(vi) Hence aquatic life has very slow rate of metabolism.

(i) The ocean is a continuous body of salt water that forms the major part of hydrosphere.
(ii) Geoscientists believe that the oceans were formed on Earth nearly three billion years ago.
(iii) It is difficult to believe that in the beginning there was no water on our planet.
(iv) In due course of time, when the Earth started cooling, steam escaped from the interior and entered the atmosphere to form clouds.
(v) At first, the clouds brought incessant rains.
(vi) The rain water filled the depressions for tens of thousands of years and eventually a super ocean was formed.