The Temple of Water is a location in Final Fantasy III. It is located west of the Cave of Tides, and it was inhabited by the Water Maidens. Other than story cutscenes, there is nothing special that may occur in the Temple of Water.
HP/MP and Revival Wellsprings are also available in the temple.
1,000 years ago, the powerful Xande created an imbalance of Light and Darkness and time froze on the Overworld. When the Warriors of the Light meet Aria Benett, the last Water Maiden, she asks them to head with her to the Temple so that she can lift the seal on the Cave of Tides, in order to restore the Water Crystal.
The background theme that plays inside the Temple of Water is "Room of the Crystal", which may mean that the Water Crystal once stood inside this shrine.