Amanchu!

Hikari Kohinata is an overwhelmingly cheerful and outgoing 15-year-old girl who has spent all her life in a small and beautiful seaside town on the Izu peninsula. On the first day of high school, she meets Futaba Oki, a slightly introverted girl who has just moved from Tokyo. Before she knows it, Futaba gets dragged along into Hikari's passion for scuba diving. And from there, Futaba gradually starts to come out of her shell, and learns how to take challenges and dive into the ocean of life.

(Source: Anime News Network)