"AquaTale? Isn't that an AU where all the characters are mermaids for some stupid reaso-"

Nope! While there is an AU called Aquatale which is like this, this is a different Aquatale.

"Then what is this Aquatale about?"

It's a prequel before the first human (Chara) falls underground. Where before the first human fell there was another called Hazel, Hazel has a Cyan/Aqua soul but isn't the one in Undertale, While Hazel fell down Mt. Ebbot she ended up in a different underground. Throughout her journey in this other underground, she meets different characters who become her party members and teach her magic spells.
There is much more but that's it's simplified.

(Note: AquaTale is a sprite comic, not a game)