Bree is a bright, determined middle school student who moves to Florida and faces her intense fear of the water when swimming becomes her only elective option. She loves math and puzzles, preferring to approach challenges methodically. Navigating a new school, she battles intense anxiety regarding sports and making friends, visualizing her fears as encroaching dark thought bubbles.

Etta is an older woman living in the apartment above Bree. As a former swimmer for Enith Brigitha Middle School, she understands the rich tradition of Black swimmers and the historical barriers of segregation. She steps in to teach Bree how to float and swim, offering patience and an approach that resonates with Bree's logical mind.

Ralph is Bree's single father. He moves their family to Florida after accepting a technology training program and works two jobs to provide a stable life. He places a strong emphasis on education over extracurriculars and friendships, though his frequent absences from Bree's swim meets create a rift between them.

Clara is an outgoing student at Enith Brigitha Middle School who quickly befriends Bree. She is determined to join the swim team and aims to win the state championship. While confident in the water, she struggles with math and relies on Bree for academic tutoring while facing pressure from her mother regarding private school applications.

Phillipa is a swimmer on the Enith Brigitha team who struggles with her speed and floating abilities. Known by the nickname "the Anchor," she uses defensive humor and occasional anger to mask her insecurities about her athletic performance.

Keisha is a wealthy transfer student who initially swims for the rival Holyoke Prep team. After her former coach dismisses her for losing a race, she joins Bree's team. She brings private-school training and an abrasive attitude to her new squad, causing friction before learning to value teamwork.

Tinsley is a competitive swimmer for Holyoke Prep. She takes pride in her school's unbroken winning streak and openly mocks the Enith Brigitha students, specifically targeting Bree when she suspects her inability to swim.

Humberto is a friendly classmate of Clara and Bree. He is involved in the school play and frequently hangs out with them at the local park, offering occasional help when the swim team needs a favor or props.

Yvette is a bank employee in Florida and one of Etta's childhood friends. She swam alongside Etta on the Enith Brigitha middle school team decades ago, sharing a historical bond and memories of discrimination.

Jamie works at a local dentist's office and is another member of Etta's original middle school swim team. She understands the social dynamics of a swim team and uses her experience to encourage the younger generation of swimmers.

Mari is a woman who works at the local diner. She was the fourth member of Etta's childhood swim team, though a past incident involving a segregated pool created a decades-long rift between her and the other three girls.