Hikari Kohinata is a cheerful 15 year-old girl who lives near the ocean
and she spends much of her time diving as a result.
On her first day of high school, she meets a teacher
who also likes scuba diving.
There’s also a 16 year-old classmate, Futaba Ooki,
who gets dragged along in Hikari’s maelstrom as soon as they meet at school.
Rocchi
So basically this is manga where the author/reader self-inserts as the teacher to watch teen ass diving?