official_art? if so, then I find it odd since usually Aqua is drawn in ways that suggests she doesn't wear panties.
That is a joke. In the manga an LN she is shown to wear panties. In one of the LN pictures the anime joke is actually addressed though as the picture hints at her wearing what looks like a sort of thong or one of those high-rise "boy-shorts" panties. Panties such as those would indeed look as if she didn't wear any underwear when viewed from the angles that the anime show us.