I have seen your movie. It's not an HK film but a chinese one. In fact, it's the first full-length chinese animation film done in wide screen. It's not in the database, but I swear for my mother that one day i will add in, because i have all its data.
The star is not a girl, but a (very cute) boy named Na Cha, who fought the evil dragons in clouds and under the sea, and almost at the end of the film he suicides to save his friends... but I won't tell the end of the story.
The title is "Prince Nezha Triumphs Over King Dragon" (Nezha nao hai, 1979) (I think it haS other titles, as "Na Cha Conquers the Palace of Sea") directed by Wang Shuchen, Yan Dingxian and Xu Jingda (three of the best chinese animators alongside the Wang Bros.). It's based in the novel "Nomination of the Gods". It's a very good animation film.
There are more versions, but in live action, about this novel (1927, 1948, 1950, 1962, 1965, 1973 and 1983), but the most famous one is from 1974, directed by Chang Cheh an with Alexander Fu Sheng as Na Cha. There are also two TV series, one in live-action and other in animation format.