Water is vital for life on our planet: Water literally starts and maintains life on planet Earth.
It cannot be replaced by any other liquid, and enzymes don't work in its absence.
Organisms wouldn't develop without water (even we are around 60% water) and the earliest life on Earth was a result of the presence of water.
The ecosystem that we enjoy is entirely dependant on H2O, and even single cellular organisms can only survive and reproduce in the presence of water.
I'd recommend checking out an article like the following if you'd like to know more: http://science.howstuffworks.com/environmental/earth/geophysics/water-vital-to-life.htm
But the simple fact is, scientists have yet to discover a lifeform that isn't dependant on water in one way or another, which is why they don't bother looking for signs of life on planets that couldn't sustain water, the galaxy is too vast and, as far we know, water = life.