Tsukiko Yano is a transfer student at a highschool where she doesn't find anyone or anything interesting. That is, until one evening when she sees her classmate Aya Utsumi swimming in the school swimming pool. After that, she can't help but see fish swimming all around her.
Yoru to Umi is a completed manga by Goumoto, first published in 2017. It blends sports and slice of life. You can read all 19 available chapters of Yoru to Umi online for free on MangaPulse; the newest is Chapter 19, added 2021-07-18. The series is finished, so you can read it from start to end in one go.