Mẫu Thượng Thiên

Title	Mother Goddess of Water
Domain	Rivers, lakes, seas, rain and anything aquatic
Symbols	Waves, aquatic creatures
Color	White
Consort	Lạc Long Quân, Dragon King (according to some sources)
Offerings	Flowers, seafood, rice, clean water (not comprehensive)

Water Mother

Mẫu Thoải or Mẫu Thủy (Water Mother) is responsible for nurturing life, ensuring good fishing yields and protecting sailors. She rules from her Water Palace (Thoải Phủ) and is, naturally, associated with anything water-related like floods, aquatic life and rain. She's often depicted as an elegant women in blue or white rboes sitting on a dragon or snake and sometimes holding a water jug or just standing on waves.

Worship

Small shrines dedicated to her are scattered along various bodies of water and fishermen often pray to her for safety (and abundance). When giving offerings, one should avoid anything red (represents fire, opposite of water) and stale or dirty water. Rituals revolving around her often include washing hands (cleansing oneself spiritually) or offering sacred water.

Domain

Her palace is imagined to be a grand, underwater kingdom with things like coral gardens, dragons, sacred fish and snakes. She's the embodiment of fertility (for beings and plants alike), logical considering water keeps life alive. However, she also acts as a destroyer with things like floods and stormsreminding worshippers of her importance and the need to respect her.

Servants

Her servants assist her in executing her will and include various water spirits and other mythical beings, most prominent of which being the Four Dragon Kings (Tứ Long Vương) who each rule over a different direction (North, East, South, West) and their own seas. They can also control the rain, floods and storms acting in the Water Mother's will.

Officials, Fairies and Other Spirits

Other servants include spirits such as the Lord of Water (Ông Thủy) and the Third Prince (Quan Đệ Tam, sometimes associated with water though not exclusively). She also has various water fairy spirits and nymphs (Tiên Nữ Thủy Cung) who dance gracefully during spirit possession rituals, some having believed to be drowned maidens who became water spirits after death. The Serpent Deities (Xà Thần) are giant, benelovent snakes who guard rivers and the River God (Hà Bá) demands sacrifices (sometimes so villages avoid flooding).

Origins

Varies across regions, as in some versions she's a dragon princess who fell in love with a mortal which led to punishment and veneration as a protective goddess. Though, a popular one says she was originally a dragon princess married to the Dragon King. But, a jealous concubine (or rival water spirit) spread rumors that she was conspiring against him by blessing fishermen too much (which weakened the Dragon King's authority) or having an affair with a mortal hero.

This enraged the Dragon King (or the Jade Emperor, overseer of the universe) and banished her to a prison, deep in the ocean. She weeped, and her tears turned into floods while her anger made storms. When her worshippers began honoring her upon seeing these disasters, she was let out of her cage and pardoned.

Versions vary, but the general jist is that she was first betrayed, then banished, then re-released and made goddess.