Water, or H20, is a compound with two hydrogen atoms and one oxygen atom. Water is a necessity for all known life and is found throughout the solar system. 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.
The composition of water is very important. Many celestial bodies have higher salinity, a different pH, or might contain heavy metals (see Salt Ocean Planet for details). Terraforming them will be hard.
Also, the isotopic composition of water (more specifically, of the hydrogen in water) throughout the universe. Earth's 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. Deuterium-depleted water may have anti-cancer effects. On the other hand, heavy water (water rich in deuterium) can cause sterility and in high amounts it can be deadly].