Water Pledge inflicts damage. It has a regular power of 50.
When two allies attempt to use Water Pledge and either Fire Pledge or Grass Pledge on the same turn, the ally moving first will not itself use a move, but instead the ally moving second will use a combined attack with a power of 150 and an additional effect immediately after it:
If Water Pledge and Fire Pledge are used in the same turn, the ally moving second will use Water Pledge with a power of 150, and create a rainbow on the user's side of the field for four turns. The rainbow doubles the probability of secondary effects taking place on that side of the field; this effect does not stack with Serene Grace.
If Water Pledge and Grass Pledge are used in the same turn, the ally moving second will use Grass Pledge with a power of 150, and create a swamp on the target's side of the field for four turns. This swamp quarters the Speed of all Pokémon on that side of the field.
If a Pokémon uses one Pledge move and a partner has selected another Pledge move, but none of its partners are able to use their selected Pledge move that turn (e.g. paralysis, Sky Drop, etc.), no move will be executed.
Water Pledge is unaffected by a held Water Gem (and will not consume it).
Generation VI onward
Water Pledge's regular power changed from 50 to 80. In addition, Water Pledge does not work in Sky Battles.
It damages an enemy. If it's used with Fire Pledge in an Alliance, you and your teammates will get the rainbow condition, and if it's used with Grass Pledge, the enemies in the same room will get the swamp condition.