Water, or H20, is a compound of two part Hydrogen and one part Oxygen. Water is a necessity for all life as we know it (Carbon-based life forms) and is found throughout the solar sytem. If you added up all the water in solid, liquid, and gaseous forms on the icy satellites of our solar system, it would equal over 40 times the amount of water on planet Earth. Terraformed worlds would need to have large amounts of water availiable to make it habitable for humans.
Water composition is very important. Many celestial bodies have a more salty water, alkaline or acid or might contain heavy metals (see Salt Ocean Planet for details). Terraforming them will be hard.
Also, there is not the same water throughout the Universe. Earth water has a relatively low deuterium concentration. It has been proven that comets have a different amount of deuterium. ESA's Rosetta probe revealed that comet 67P has far more deuterium then Earth's oceans . It has been proven that deuterium-depleted water can increase lifetime, delaying aging. Also, it can be useful in treating cancer . On the other hand, heavy water (water rich in deuterium) can cause sterility and in high amounts it can be deadly .