User blog comment:MagnoCosmos/Celestial Spirits Ranking/@comment-2214433-20150907165840/@comment-7418318-20150908211740

Aquarius can't produce water?

Then how did she fill the dome in which Lucy fought against Succubus-Juvia and Vidaldus Take up with water?

Then there's also the Tartarus arc, where she also brought in a lot of water, which then disappeared again when her key was broken.

Or the cave in the Tenrou arc where Lucy and Cana fought Freed and Bickslow. There was only tiny puddle of water and Aquarius still unleashed a torrent.

She can both controll water and call-in/summon/create more of it.