At the Dragon Palace, after having a conversation with queen Otohime, Amaterasu would be given the Shell Amulet to open the door to the Water Dragon's garden. Do not head there yet; instead, go down the corridor to the right of the palace's main hall, in the end is a door with a Dragonian guard blocking it. Talk to her, and upon recognition that Amaterasu is the holder of the Shell Amulet, she will be allowed to proceed further into the garden. With Digging Champ, dig up the stone slab in the center of the room, and Amaterasu will be informed that underneath it is a dried up spring, and she will be requested by one of the dancers to help them restore it. Accept the request, and a digging minigame will ensue, only that Amaterasu starts at the bottom of the area, and has to make her way up to the top to lead the dancer down to the bedrock. Once the dancer is down to the lowermost level, she will mark a specific block of rock that could be dug up, restoring the spring. However, the dancer will further inform Amaterasu that the spring actually swirls, and to pray for it to be the same again, the dancers will circle around the spring. Draw a circle or a swirl in the center of the spring, and it will be fully restored to be a Mermaid spring, and Nuregami will also grant Amaterasu Fountain.
Spoilers end here.
Fountain is the only Celestial Brush technique in the entire of the Ōkami series to have two different brushstrokes that yield a single result. All other techniques with more than one brushstrokes have different outcomes for each drawings.