Crunchyroll streams the manga's anime adaptation, and it describes the story:
Kohinata Hikari and Ooki Futaba have together found joy in many things through scuba diving and the most everyday of events. Hikari was born and brought up in a small oceanside town on Izu, and Futaba moved there from Tokyo, but the two polar opposites met at Yumegaoka High and instantly hit it off. The always-reserved Futaba found her perfect foil in the cheery and optimistic Hikari, and their friendship gave her the courage to take the first step and experience many of the smaller joys in life. She also took the Open Water Diver exam and passed, and now she has her sights set on the Advanced Open Water Diver certification.
This once-in-a-lifetime high school life is still only beginning. Their magical days filled with wonder won't end as long as they keep walking together.
Amano launched Amanchu! in Mag Garden's Comic Blade magazine in November 2008 after having finished Aria earlier that year. Mag Garden published the manga's 15th compiled book volume last December. The 16th volume will ship in February 2021, and the 17th will ship next September.
Source: Kozue Amano's Twitter account