If you're looking for manga similar to Ritou no Umi, you might like these titles.
Ji-Eun, a children's storybook author goes to Guam for her friend's wedding. There, she develops a strong fascination with the ocean and decides to become a diver. As a scuba diver and free-diver, she experiences the universe beneath the earth, a new life, and people who love the sea, and explores lost dreams, love, and the meaning of life.
Salvors are men responsible for towing wrecked ships and handling the cargo. They are different from rescue teams as they usually ask for money before offering help. Nanba Rintarou's father was the owner of such a company, but Rintarou really hated the way they operated and decided to join the Coast Guard. However, Rintarou's father died right after Rintarou completed his training. In his will he asked Rintarou to take over his company. What's worse is he left his son a debt of 1.5 billion yen...
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.
Nakabaru Taiyo came to Okinawa in the pursuit of beautiful ladies and the blue sea... But while trying to hit on a cute girl, she told him that her mother hated him... And then a wonderful story of scuba diving, beautiful women and adventure begins!
Chappy's only strong point is his cooking. While working at a hotel on a quiet island he meets Natsu, who shows him a whole new world in Diving. Together they search for the Pink Napoleon fish that is said to bring happiness.
It's been years since Marie was saved from the ocean, and has lived on a solitary island ever since. Her first and only friend is Canu, a strange creature from the sea. But she hasn't seen Canu in awhile, and ventures out to find it...