Question

why do sea mammals drink salt water as you know salt water is not drinkable? why can't freshwater fish can't survive in salt water and vice versa ?

Answer 1

Sea animals drink sea water beaause they have evolved in the sea environment and therefore the concentration of salts in their tissues is similar to that of the sea.

1. Water concentration inside a fish is higher than in the ocean itself because the ocean is so salty. As a result, most saltwater fish constantly lose water through their gills and skin.

2. Because the fish is losing water, it must drink a lot to stay hydrated-but salty seawater is the only water around.

3. To get rid of excess salt, the fish's kidneys pump lots of salt into its urine.

Second part of question :
Fresh water does not have enough salt concentration to satisfy the survival of saltwater fish.Therefore the fish's hypertonic cells will become hypotonic which will not mix well with the body structure of salt water fish, and they will die. 

Third part of question :
if our freshwater fish were placed in salt water, the water molecules contained in the fish'sbody would pass through the cell membranes of the fish, causing the fish to lose water.