If you're looking for anime similar to Nanatsu no Umi Nanatsu no Sora, you might like these titles.
The tomboy Benio is forced to marry the lieutenant Shinobu, who is from a wealthy family. At first Benio doesn't get along with the distinguished way of life at all. When Shinobu is sent to fight in Russia, Benio makes her own way... as an emancipated woman in Tokyo of the 1920s!
The story begins in Vienna in 1907, as the city holds a lavish birthday celebration for its beloved Princess Flora. The princess entertains suitors from around the world who have come to propose marriage. Also on hand for the celebration is Phoenix, a handsome, gentlemanly jewel thief who has his eye on stealing the princess's ring containing the gemstone "Smile of the Amazon." Upset with Phoenix's brash manner, Princess Flora drops the ring into a cooked fish and throws it out the window. Meanwhile, a young teenaged orphan named Honey Honey is working as a waitress. Her pet cat, Lily, spies the discarded fish and proceeds to eat it whole, thus swallowing the ring. Since Princess Flora has declared that whichever of her suitors successfully returns the ring to her shall become her husband, the suitors and Phoenix immediately fan out all over the city pursuing Lily and her owner, Honey Honey.
While Randy was walking through the Swiss countryside he finds a small girl, who seemed to be the only survived person from an airplane crash. The little girl didn’t remember anything, and Randy decided to take care of her and he called her Judy. The both go their childhood together. A few years later, Judy, now 16 years old, wants to find her past. With the help of Randy, she leaves to research her origins. Her single clue is a song that she keeps hearing in her head, the song is called Alpine Rose. At a time of world war, these two young people will have to overcome many obstacles to arrive at their goal in this time of war, with having only each other to comfort and support. the depth of their love.