Water Bubble halves the damage dealt to the Pokémon with this Ability by Fire-type attacks and doubles the power of Water-type moves used by the Pokémon with this Ability.
It also prevents the Pokémon with the Ability from being burned. A Pokémon with Water Bubble will be cured of burn if it is burned while in battle (such as if it gains the Ability by an effect like Skill Swap, or is burned by a Pokémon with an Ability like Mold Breaker).
Outside of battle
Water Bubble has no effect outside of battle.