All cellular processes take place in a water medium. All the reactions that take place within our body and within the cells occur between substances that are dissolved in water. Substances are also transported from one part of the body to the other in a dissolved form. Hence organisms need to maintain the level of water within their bodies to stay alive.
Terrestrial life forms need fresh water for this since their bodies cannot tolerate or get rid of the high amounts of dissolved salts in saline water. Plants require a large amount of water for carrying out transport through the xylem.