The Water of Hermes is a rare item found at Mercury Lighthouse in Golden Sun. It requires the use of an Empty Bottle, found in a chest in the Old Couple's home in Imil, found near the entrance of the town.
If used in or out of battle, this item restores one character to full HP, the same effect as a Potion. Once used, the player is left with an Empty Bottle, which can be sold or refilled. Hermes' Water's usefulness is somewhat limited, as the trip to the Imil region generally takes a long time and is a dead-end; common Potions and Vials function practically the same way. In the end, it is always good to refill the bottle when the opportunity arises, as it is an unlimited, free supply anyway.
Being Artifacts, both the Water of Hermes and the Empty Bottle can be sold and later repurchased. The Water of Hermes can be bought for 2000 coins and sold for 1500 coins. The Empty Bottle can be bought for 1000 coins and sold for 750 coins.
Hermes' Water returns in Golden Sun: Dark Dawn as an important item given by Briggs at Port Rago after it had been given to him by Piers. Since the Imil region cannot be accessed in Dark Dawn, it would be impossible to refill the bottle, so logically you cannot use it on your party. It is used in Kolima Village to heal the sick Dream Tree, which makes Pewter appear and makes Kolima Forest explorable.