After accepting the text titled Rusalka from librettist Jaroslav Kvapil, Dvorak spent 6 months working on the opera until its completion on November 27, 1900. The poem's chief source of inspiration can most notably be given to Erben's ballad's in his Bouquet. Premiering at the Prague National Theatre in 1901, Rusalka is attributed as being Dvorak's his "most wagnerian" opera he ever composed. Being through-composed, he utilizes the art contrast with respects to the two differing world's within the opera: the human and supernatural. By doing so, he embodies Kvapil's portrayal of being incapable of uniting the two opposing world's together.
Dvorak utilizes a proper amount of leitmotifs in his opera, most importantly being the leitmotif he uses for the title character Rusalka; however, one would assume that the main theme of the opera is representative of the main character but it is his use of his "water magic" theme that takes the cake for being most prominent in the opera. It is represented by it's romantic motif of frustrated yearning, being omnipresent both in the music and the plot.
The Act I synopsis of the story goes as follows: Rusalka, a water-sprite, falls in love with a human prince who hunts near the lake in which she resides. Asking a witch to turn her human, she gives up her voice and her existence as a water-sprite. If she turns out not able to keep the love of a human she will be damned and will have to return to the depths of the lake. Unwaiveringly, she meets the prince in her human form and he falls in love with her, taking her back to his castle, thus ending the act. Within this act, Rusalka sings a song to the moon, asking it to reveal her love to the prince.
https://en.wikipedia.org/wiki/Rusalka_(opera)
http://www.antonin-dvorak.cz/en/rusalka