Theory:

Water (Magical liquid)
The versatility of water is essential to live organisms.
Water exists even on other planets both within and beyond the solar system. Water covers 71% of the earth surface. It is known as the universal solvent.
“If there is magic on this planet, it is contained in water.”-Loren Eiseley
Characteristics of water:
 
shutterstock_1595730745.jpg
  • Hydrogen and oxygen make up water with hydrogen bonding.
  • It exists in gaseous, liquid, and solid states.
  • Water is tasteless, odourless liquid at room temperature.
When in large amounts, water has an intrinsic blue colour caused due to the slight absorption of light at red wavelengths.
“If behaviour defines things, then water is an altruist.” - Wilkins
shutterstock_1496535638.jpg
 
The water molecules (H2O) are simple, but the physical and chemical properties of the water are extraordinary. Also, these rare properties of water, such as its thermal density, high specific heat, viscosity, and liquid-solid characteristics, ensure that life thrives.
 
shutterstock_560960176.jpg
 
For example, almost all elements have their solid-state denser compared to liquid as we know due to the arrangement of atoms in them, but in the case of water, we can see ice (solid) floats on water(liquid).

ice-2637363_1280-w1280.jpg

If the ice is dense, imagine the ponds and lakes would freeze completely, killing all the aquatic living organisms present in them; due to this unusual characteristic of water, the ice acts as a barrier to protect the organisms present in them.
 
The floating of ice (solid) is due to the cage-like structure, which has a lot of empty space, making it less dense than its liquid, where more molecules are present in them, making it denser.
 
icicles-3157278_1280.jpg
 
We know that all cellular processes take place in water which acts as a medium. All the reactions that occur within our body and the cells occur between substances dissolved in water.Transportation from one part of the body to another takes in a dissolved form.
 
ybhy_i0po_160907.jpg
 
The aquatic animals breathe oxygen dissolved in water, and there is also oxygen present in the soil for organisms that live under the ground.
Fact: We know that the human body (adult) contains 60% of water.
Thus, all organisms need to maintain the level of water within their bodies to stay alive. Terrestrial life-forms (animals that live on land predominantly) require freshwater because their bodies cannot tolerate or eliminate the high amounts of dissolved salts in saline water (saltwater).
Reference:
https://www.vecteezy.com/free-vector/aquatic-animals">Aquatic Animals Vectors by Vecteezy