Aria the Avvenire: Sono Aitakatta Anata ni... (Aria the Arrived: To You, Who I Wanted to Meet...)
The story starts with Ai practicing on the gondola with Akari. As they travel through a canal, they meet Aika and Alice. All three have not had much time to meet since becoming Prima undines. Considering even the little time they spend together a "miracle," Akari remembers a certain incident and relates it to Ai.
[Source: Anime News Network]