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.