Life on earth (and presumably elsewhere) depends on water. The biochemistry of life takes place in aqueous solution. Even organisms that live on dry land are mostly water (cells are liquid internally). Water has several key properties that life depends on.
The water molecule (H2O) is slightly polarized. The hydrogen end is positive and the oxygen end is negative. This makes water a powerful solvent (it is commonly known as a ‘universal solvent’). This is important because once in solution, chemicals can easily interact.
The two hydrogen atoms form an angle of 105° with the oxygen atom. This angle allows water to form a loose lattice with each molecule bonding to three or four others. These are very weak bonds that are constantly breaking and re-forming. This gives water some remarkable properties. First, it’s transparent to visible light. This lets sunlight penetrate the top few meters of the ocean, giving photosynthesis some working space. However, water absorbs harmful radiation like UV and microwaves, which would damage or even prevent life. Next, water has a very high specific heat, which moderates (buffers) the earth’s surface temperature, keeping it from getting too hot or too cold – again, important for life. Next, water actually gets less dense as it freezes into a solid, which is strange. Ice floats. Imagine what would happen if it didn’t. Layers of ice that formed on the surface would sink to the bottom of the ocean, where they they would stay frozen. Eventually, the whole ocean would freeze solid, and life wouldn’t be possible at all. Next, water has a high surface tension and resulting capillary activity. This allows water to be carried up plant stems, against gravity. Next, water can be either a base or an acid in chemical reactions.
Water is so fundamental for life that it’s what we look for whenever we search for life on other planets or even in deep space. The temperature range where water is liquid is the ‘sweet spot’ for life.