The three siblings (Riku, Umi, and Sora) mean "Land, sea, and sky" in Japanese.
Backstory - During the Korean war Japanese merchantmen and crew played a logistical role by ferrying men and supplies, and as such incurred losses in men and material.
There's a French theme in this movie. The clubhouse Quartier Latin refers to the Latin Quarter in Paris. On one of the desks, there's a French-Japanese dictionary. Umi's nickname is Mer, both of which means "sea" in Japanese and French respectively. Poppy Hill is supposed to be named Coquelicot Hill, coquelicot being French for "poppy". Both Poppy and Mer were removed from the official English subtitles for the movie.
The flags that Umi raises mean U and W in international flag language. The two letters mean, "I wish you a pleasant voyage". At one point, Umi raises the flags that spell out Hokuto, which Shun translates for the audience. Shun's flags on the tugboat say UW MER, which are seen again in Sachiko's painting.
Aoi Teshima does the voice for Umi's friend, Yuko. She also sings the ending theme, "Summer of Farewells - From Up on Poppy Hill". Aoi Teshima was also the voice of Therru in Tales of Earthsea and sang Therru's theme song too.