In chapter 14, Aya takes Tsukiko to an aquarium. Aya is trying to form a connection with Tsukiko. Doing things in the template of a typical friendship turns out to not be the right way to connect with Tsukiko though.
Posting about some of the manga I read. I read manga in Japanese.
Utsumi Aya enjoys swimming and Yano Tsukiko enjoys watching her. Tsukiko associates Aya with fish imagery. A flashback in chapter 4 shows Tsukiko being taken to an aquarium as a child and clutching a fish plushie, which seems to suggest an emotional significance to Tsukiko viewing Aya as a fish.
Localized by MangaPlaza as The Evening and the Sea.