The user soaks the target with chilly water that harshly lowers its Special Attack stat. Water-type targets receive a boost, however.
The user douses the target with water, completely saturating it and interfering with its ability to use special attacks. Water-type targets, on the other hand, enjoy the shower of refreshing water and will use it to fuel their own moves.
If the target is a Water Pokémon, raises the target's Special Attack by two stages. Otherwise, lowers the target's Special Attack by two stages. Water-type targets with Water Absorb or Storm Drain will have their Special Attack raised by Waterlog's effect; non-Water-types with those abilities (e.g. Maractus) will not have their Special Attack lowered, however.