Vodyanoi seldom come on shore, but when they do, it is typically thought to be with evil intentions toward humans. They avenge real or imagined injuries, retaliating against pearl divers who trespassed on their territory or simply going on crusades. Warriors make up the core of the underwater army and while they must wear breathing gear on land, they are at least the equals of their human counterparts in the art of combat.
In the short story A Little Sacrifice in Sword of Destiny, the residents of the underwater city of Ys are not called "vodyanoi" but simply "People of the sea" ("Morski lud"), but it seems quite clear that the creatures in the game were based on this story.
Season of Storms and Fox Children
In Polish (original) edition of the Season of Storms novel, crew of the Prophet Lebioda have encountered a large creature named "vodyanoy" ("wodjanoj", pronounced almost same as "vodyanoi") which destroys the ship's rudder. It is not certain if this is the same race which inhabits Ys since it was unnamed in A Little Sacrifice short-story. However, the Fox Children, a comic adaptation of one of the chapters of the novel, contained both variants: human-sized and the larger one.
In The Witcher computer game
Written sources confirm that the fishpeople are very religious and hold their priests in high esteem. If only they could restrict themselves to praying at their underwater altars! Unfortunately, during vodyanoi raids on shore, the priests support their warriors with religious spells. In these instances, an honest, hard-working witcher must put in that extra effort.
The brickmakers in the Swamp, under the guidance of Vaska, worship the "Water Lords", which Geralt suspects to be vodyanoi. Dagon worshippers are not actually described in the game. That term is used to describe the "bad" vodyanoi.
Developer CD Projekt's characterization of the Vodyanoi taken from the monsterbook, which was enclosed with the Collectors Edition of the computer game The Witcher for Poland, Hungary and the Czech Republic:
The race of the underwater creatures described in the short story 'A bit of Devotion' is undoubtedly intriguing: a civilization thrives in the depths of the sea, developing in parallel to those on land. This motif provided an opportunity to tell a tale of two worlds colliding. In the game, relations between the vodyanoi (inhabitants of the Underwater City whom humans contemptuously refer to as "fishpeople") and the villagers of Murky Waters are tense. A mysterious deity named the Lady of the Lake and, of course, our protagonist Geralt of Rivia, step in to mediate.
The sketches depict members of two vodyanoi castes as well as acolytes of the menacing water god Dagon. The artist concentrated on clothing and accessories in an effort to emphasize the distinctive nature of the underwater civilization. The priest wields a staff and wears a high hat with straps of leather falling on his shoulders — objects resembling the staff, miter and vestment of a bishop. His mask and the bracelets on his hand emphasize that the vodyanoi know metallurgy and view metal as a rare and valuable rescource. The remaining elements of this ceremonial outfit feature more common materials: his belt is made of oyster shells and stones, the straps of his vest of leather, his stone-encrusted breast-plate of wood. The stones have a religious significance connected with the cult of the water god known as Dagon.
The vodyan warrior wears a lighter outfit. His armor does not restrain his movement, enabling him to fight ably both under water and on land. As vodyanoi absorb oxygen naturally only from water, he wears a mask that allows him to breathe on land. Modeled after old gas masks, this breathing apparatus grants the warrior and all vodyanoi a singular appearance.
The straps and plate on the warrior's chest are also lighter. They are not armor, but (as in the case of priests) religious objects. The netting evokes connotations with water, and was probably looted off of fishermen during a raid.
In the early development stages, vodyanoi — as opposed to other monsters — were supposed to use weapons, hence the harpoon in the sketched warrior's hand. This idea was ultimately abandoned: even though they are thinking creatures, fishpeople use their claws in combat.
The third sketch depicts a vodyan dissident and worshipper of the menacing water god Dagon. The schismatic and his comrades emphasize their autonomy by wearing a different mask, one modeled after a knight's helmet. The breast plate is larger than that worn by warriors, because Dagon's acolytes consider themselves to be priests.