Water

Water in Metroid games is a completely harmless substance often encountered by Samus during her adventures. It does not appear to be any different from Water on Earth, and is mostly found in aquatic, underwater areas (sub-zero and other, non-aquatic/cold areas are also known to have quantities of water).

The life-support systems in Samus' Power Suit allow her to survive underwater almost indefinitely (though it is unknown how it affects her when she is not in her suit). However, she cannot swim, nor can she move quickly or jump high underwater before she gains the Gravity Suit in most games (the Gravity Boost provides a similar effect in Metroid Prime 2: Echoes). This also applies to her movements in Acid, Lava, and any other liquids in the games.

Hazardous Water
In Metroid Fusion, when Samus first arrives in Sector 4, several rooms have water that has been electrified by broken power nodes. Upon reaching the Pump Control Unit, Samus lowers the water level to below the broken nodes, rendering the water safe to travel in.

In Metroid Prime, the water of the Chozo Ruins was poisoned by a massive plant beast known as the Flaahgra, which had grown to immense size and power after a Leviathan struck the planet. The poisoned water was similar to Acid in nature, in that it drained Samus's health upon contact. The water was purified after the Flaahgra's defeat.

Due to Dark Aether's highly toxic nature, Samus found Dark Water there instead of normal water. This substance was similar to the poisoned water of the Chozo Ruins but was far more potent and an opaque, murky purple colour. The effects of Dark Water could only be nullified with the use of the Light Suit.