The Hydrus Coin can be found in the Upstream section of the Cortica River before you even swim into Spirit Falls. It's at co-ordinates B-3 and its "Shine Spot" is to the West as you swim upstream, almost on the "wall" of the river, and the Shine Spot also has two blue Guppies swimming around.
- The Hydrus Constellation engraved upon this coin is meant to represent a water snake. It is the counterpart to the larger Hydra constellation.
- The fact that Hydrus represents a water snake is reinforced by the fact that it is next to the constellation Eridanus, representing a river.
- A myth associated with the Hydrus constellation is that Corvus, the crow (or raven), was going to fetch water for Apollo using his cup (the constellation Crater). Corvus stopped to eat figs along the way, but lied to Apollo, saying that a great snake (Hydrus) kept it from the water (when in reality, Hydrus was a random water snake that had done nothing wrong). Apollo saw past the bird's lie, and in return, flung the three objects into the sky.