In the first episode of Origination, Akira not only manages to keep the boat steady while catching a misbehaving customer in mid-air, she does it in heels.
Any time Akari utilizes her backwards rowing. One manga panel even depicts her with a Let's Get Dangerous! attitude about it.
Mixed with Heartwarming: Before Alice really befriends Akari in her introductory episode, she sees Akatsuki teasing her about her "sideburns" and gets about ready to wallop him with her oar for harassment! This is even before Alice starts calling Akari by name.
The scene itself is a positive Tear Jerker with the awesome moment happening in hindsight because, not only was Alice the first of the main trio to be promoted to Prima, she was the first Undine ever to promote straight from Pair to Prima.
The flashback in episode 11 of The Animation when Alicia and Akira find out just how good of a singer Athena is. She draws everyone who is on the street into her audience and earns a massive applause.
Episode 3 of The Avvenire has the 3rd generation Undines secretly organizing a reunion for everyone with Ai counting entirely on the arrival of the Acqua Alta to free up everyone's schedules. Unlike Akari, she isn't drawn to and simply recognizes miracles, she actively relies on and utilizes them as part of the plan.
Cait Sith saving Akari from the ghost in the cemetery on Isla San Michelle. It is also the first time in their many encounters where he touches her directly by caressing her cheek. For a mystical being that is equivalent to a god, that is respect.
Athena's third time helping Alice with her Self-Imposed Challenge of stepping only on shadows on the way home: Alice trips and looks about to fail when Athena sculls her gondola at least 5 meters in a matter of a couple of seconds to provide a shadow for where Alice's foot lands.
The entire Marriage of the Sea ceremony in a manga-only chapter, showcasing the grace and beauty of all of the Undines in and why they are considered the face of Neo Venezia.