Water

Water, chemical formula H2O, is a common chemical substance. Its chemical make-up is a bond of two hydrogen atoms and a single oxygen.

Universe and lore
Water is found throughout the galaxy on a number of planets with a variety of climates. Water on these planets are mainly found in large bodies. Shield worlds and Halo installations, both built by the Ecumene, also contained large water bodies. Some beings on these worlds, like the Sea worm or the Sanghelios creature, live predominately in the water.

Sentient species have utilized water for a number of purposes. Humans and Sangheili have both developed vehicles specifically to traverse those water bodies. Human and Covenant engineers have also used water for electricity and Plasma generation, like on Reach or on Buwan. Humans also sell forms of water like with H2Oh! Vitamin Water.

The Global Water Campaign and its GWC Trans-African Pipelines were designed to transport the still relatively clean melt water from Mount Kilimanjaro across the entire African continent. Multiple facilities of the GWC, including A2 Hydroelectric Facility and Voi Municipal Water Pump House 03, were sites of combat during the Battle of Earth.

Gameplay
Water is seen in plain sight during the majority of all the Halo games. It appears in six of the ten levels of Halo: Combat Evolved, nine of Halo 2's levels, in three Halo 3 levels, and in seven maps in Halo: Reach. It is further seen in Halo Wars 2, Halo 4, and Halo 5: Guardians.

In Halo 3 and Halo: Reach, bodies and objects will float on water for a while before sinking.

Water is often utilized as a death barrier in some maps and the player is killed upon hitting the water or when hitting the water becomes inevitable. However in some cases on Forge World and Forge Island, there are a few water areas that won't kill the player at all.