Aqua can be used once you have a level 3 castle through the mage to upgrade your equipment. Don't drink it.
I would advise you not get to interested im this game. You will just get let down again and again. This game was so good in 09-10. Now Its essentially a cash cow for ata to develop other games
I wouldn't want someone to waste valuable time on a game that isn't worth it. The game wont die from this advice. It will die from userd leaving due to dissatisfaction caused by developers.
As has already been said aqua and inferno are used to enchant equipment in the mage which is unlocked once you have a level 3 castle. If you have any more questions please feel free to wall me /lock