Water Absorb is an ability introduced in Generation 3. So far, 5 Pokémon have this ability.
When the Pokémon is hit by a Water-type move, its HP is restored by 1/4 of its maximum HP, and the move will have no effect on that Pokémon. It can be seen as a Water counterpart to Volt Absorb.
Water Absorb has no effect outside of battle.