Though water seems ordinary to us, it is composed of a unique combination of properties that are critical to carbon-based life arising in the first place, and the same laws of chemistry and physics apply to all parts of the universe. Biochemical reactions require a fluid to operate in, and everything will dissolve in liquid water to some degree. Enzymes—proteins that speed up chemical reactions sufficiently to make some form of life possible—work only with water. The list of properties goes on and on. The ubiquity of liquid water (not water vapor or ice, which are widely found on other celestial bodies) on Earth is what has allowed life not merely to exist but to flourish. Non-carbon-based life (in which a fluid such as liquid ammonia or methane is the critical substance) is technically possible, but only in theory. There are other hypothetical forms of biochemistry, but the likelihood of any of them being a reality is extremely slim due to the properties of the elements involved. So the search for liquid water makes sense. It’s by far the most likely road to finding life, which will be wildly different, regardless.