If you're looking for manga similar to Otohime Diver, you might like these titles.
The ocean holds many tales. Tales of mermaids, of underwater kingdoms, and of fantastical beasts lurking in the deep. Spurred by these tales, Kazuya sets out to find the truth, but what he finds is a baby?! Not just any baby, the baby Kazuya finds is Triton! The last of a once mighty and proud race of people from the sea. Triton seems human, but unusual and terrible events begin to happen with all those around him. Who is Triton, and why does he bring misfortune to everyone around him? From the imaginative mind of Osamu Tezuka, creator of the beloved AstroBoy series, comes his original and thrilling take on the legend of Atlantis: Triton of the Sea!
In a post-apocalyptic world covered with water, Wadatsumi, a person who lives in what appears to be the last piece of land, finds a mysterious girl dragged by the waves of the sea. Where does she come from? Are there other lands in this world?
When his ship is attacked by pirates, Archer is left floating adrift in a violent sea. But the young urchin is nothing if not resilient. He soon finds himself sailing with the infamous Cannonball Joe aboard "La Revancha," the fastest ship on the East Coast, in search of a legendary cache of hidden treasures. Their dangerous quest will lead directly to a showdown with the "Hoboken," one of the most feared pirate vessels ever to sail the Jersey shores, and its reckless captain, Lee. But there are far worse things than pirates that call these waters home...
At the end of the 21st century, the earth is covered with Human/fish hybrids. Water almost completely surrounds earth and so these hybrids known as "Fish-halfs" roam the waters. A girl named Guppy is rescued by a hybrid named Shark, who wants nothing more than killing off the navy (who try to control him).
Shachi, a young fisherman who lives in an peaceful island, is involved in a tragic event that reminds him the death of his father, which happened years ago.The boy then decides to start a journey in the mysterious ocean.
Retired deep-sea diver Albert goes on one final job to recover a unique, once in a lifetime treasure, which is said to be inside the safe of a U-Boat, 620 meters under sea. Together with his old team, the treasure hunters known as DEEP, the mission to recover the safe begins...?
Link used to live a carefree life on a tropical island with his grandmother and sister. However, when his sister is kidnapped by a large bird, Link joins the crew of a pirate ship and sails forth on a grand adventure. Along the way, he learns how to salvage items from the ocean, meets a talking dragon ship, gets his first sword and becomes acquainted with odd new friends such as Tingle, amongst other things. What else will Link discover on his quest?