There’s no canon answer in The Swan Princess for that detail, so the safest answer is: she probably would have used whatever private chamber or outdoor area Rothbart’s castle provided, but the film never shows it. The story focuses on her captivity and transformation, not everyday logistics.

What the movie shows

Odette is taken to Rothbart’s castle lair and turned into a swan, and the plot moves on from there without depicting practical needs like sleeping, eating, or bathroom use.

Most likely in-story possibilities

  • A guarded chamber inside the castle, if Rothbart kept her in human form at any point.
  • The lake or surrounding area after she became a swan, since the film’s magical setting shifts away from ordinary human life.
  • Off-screen handwaving, which is the usual answer for animated fairy tales when realism would distract from the plot.

Why there’s no explicit answer

Children’s fantasy films usually skip bodily-function details, especially when the character is under a curse or magically transformed. In this case, the movie gives enough to understand the captivity, but not the day-to-day mechanics of it.

TL;DR: It’s not explained in the film, and the most reasonable assumption is that it happened off-screen in whatever way the castle or magic allowed.