Azurus aqua is one of the less-utilized flowers within the Herblore skill. It is a required ingredient in 6 different potions. It is used to craft all tiers of Prayer and Stamina potions, as well as Unstable potions.
You can find Azurus aqua in Varrock and Lumbridge as well as some other spots.
The best spot to gather large amounts of Azurus aqua is north of the West Varrock bank, near the castle walls.