But the ship that Anna and Elsa's parents took was the ship in The Little Mermaid. And wasn't Little Mermaid, like, thousands of years after Frozen? Nice theory, but you need to read some other theories first.
Some fans say that the song is describing Elsa because of her ice powers and it can be true. But when one remembers Anna’s statement to Hans at the near end of the movie, "The only frozen heart around here is yours," they would say that the song is referring to Hans, especially at the song’s conclusion "Beware the frozen heart!"