Why do so many character names start with あ
Aria and its precursor, Aqua, came about because the author, Amano Kozue, wanted to write a story about her experience in Venice and her favorite pet cat at the time, Aria (title drop!). In fact, according to the Japanese Wikipedia article, the manga was introduced as 「水と猫の物語」, a story about water (aqua) and cats, in its early serialization.

Because Amano loves cats so much, she incorporated many cat-related elements into her story: Cait Sith, Night on the Galactic Railroad and Alice Carroll as a homage to Alice's Adventures in Wonderland which features the Cheshire Cat. There, another character whose name doesn't start with あ; the cat mascot of Himeya also doesn't have a name that starts with あ (Hime happens to be the name of one of Amano's cats).

So yea, sharing the same initial with her favorite pet cat, as well as having Alice as a main character and aqua and acqua alta as an important theme, are probably enough to set the mangaka on a naming craze.

References


http://ariapokoten.sakura.ne.jp/