Why only water in liquid form is essential to live?
A layman's view of the problem : 

Let's assume that water in liquid form was not necessary and organisms could survive on solid or gaseous water. What would some of the difficulties be to sustain such a life form?

Solid Form of water


How would the organism move without breaking or tearing itself apart? How would essential substances be transported from one part to another of the organism body?


Gaseous form


What would be the size of the organism? Gas has a much bigger volume than water.
In order to transport nutrient molecules within the a gaseous body, a huge air pressure needs to be generated. Also, how will gravity be overcome while transporting these molecules? A lot of energy will be required to maintain such pressures.


I guess nature (which always takes an optimal path) asked such questions and found that water in it's liquid form was the most suited to support life on Earth.

It need not be water alone. Other liquids may be able to support life as well. Only, we haven't discovered them yet. But whatever the substance, it's liquid form would seem to be an optimal solution for life.