When the ability is hit by a water-type move, its HP is recovered by 1 quarter of its full HP and the move will have no effect. If the ability holder is already at full HP, the move will simply have no effect. Water Absorb does not activate if the Pokémon is already being protected, such as with the move Protect.
If the ability holder is holding an Absorb Bulb or Luminous Moss, Water Absorb activates before the held item's effect and its consumption.