Basic ideas of post-colonial lit included here: The subaltern (oppressed and voiceless) and transnationalism (border societies, border crossings).
1. Merfolk are, by nature, a transnational group: Being half human and half fish, they have the ability to exist between these two worlds but never quite be an integral part of either one. King Triton has rejected the human aspect of his origins and commands his followers to do the same. While they have many fishy friends, they are not truly fish but are seen as something greater, as demonstrated by the deference shown them by (most of) the creatures of the deep. Thus, they have set up their own nation, Atlantica, within the ocean realm.
2. Ariel's grotto as a transnational space: Ariel, in opposition to her father, actively seeks out her human origins. Long before Prince Eric enters the picture, her curiosity about the human world leads her to intensive study including the collection of artifacts and anecdotes via her transnational contact, Scuttle. (see 2.5 below) It is within her transnational space, embracing both her aquatic and human roots, that she feels "at home."
2.5 Scuttle as a self-deluded transnational figure: He believes he understands the human world through his observations but really has no clue. Living proof that living somewhere doesn't make you an expert.
3. Part of your world: Ariel recognizes that, study as she might, she can never truly understand the human world without living among humans. While she has clearly learned a great deal of their language ("What's that word again? Street!"), she recognizes that something is missing, saying "I want more." She is not speaking of adding more thingamabobs or dinglehoppers to her collection but of gaining intimate understanding of the human world. This desire becomes even more keen with the entrance of Prince Eric and her ensuing infatuation.
4. Ariel in exile: King Triton learns of Ariel's treasure(d) grotto and misunderstands it as a subversive space rather than one of expression and exploration. As any number of colonizing and governmental forces have done through the ages, Triton destroys Ariel's transnational home, forcing her to choose between her origins. Having just been betrayed by all that she associates with her aquatic roots and feeling that she has nothing left there, she chooses to cross the border.
4.5a Ursula as the Coyote: She facilitates Ariel's border crossing but, as many migrant smugglers, extorts a heavy, manipulative price.
4.5b "Don't underestimate the importance of 'body language'. . . Yes, on land it's much preferred for ladies not to say a word:" In addition to being an illegal immigrant (for whom the border patrol will come in 3 days unless she can get her green card), Ursula points out that Ariel is and will be subaltern in her position as a female in a "man's world." Being unable to speak (both figuratively and literally), Ursula proposes that Ariel's only potential for power and influence within a patriarchal human society is her sexuality.
5. Voiceless: Ariel may have felt powerless under Triton's rule, but she now finds herself across the border, truly subaltern and voiceless. By a stroke of good fortune, she finds herself being ushered into what Angel Rama called la Ciudad Letrada (the Lettered City) but it is quite obvious, mainly from her initial awkwardness with human society caused by her incomplete knowledge, that she doesn't really "belong" there. She is quite clearly a guest of this society, not a member.
6. Deportation: She begins to adapt quickly to rules of this new society but every moment she spends adapting brings the border patrol closer to her door. She is caught and deported before she can secure her green card. While she may have regained her voice, she certainly has lost any shred of influence she may have possessed in either of the two worlds, having sacrificed all she had to the Coyote.
7. My hero: Ariel, disenfranchised and powerless, lies at the bottom of the whirlpool. She can do nothing for herself but feebly attempt to survive. It is not she but Prince Eric, representative of the Lettered City, who holds the power to save the day, release her from her prison and the bonds into which she placed herself and restore order to the world.
8a. "Get everything you wanted. . .": In the end, Ariel is granted the opportunity to remain with her prince in the Lettered City. The use of the word granted is carefully chosen here as she does nothing to "earn" her place in this world but, rather, is gifted her desires by Prince Eric and King Triton, both patriarchal authority figures.
8b, ". . . lose what you had.": As Triton says, "There's just one problem left. . . how much I'm going to miss her." She gains her voice in her new nation but at a tremendous cost, perhaps even worse than that exacted by Ursula: She is forever cut off from her society of origin. As with any subaltern seeking to have a voice in the Lettered City, she must adapt to their discourse, their way of life, and sever her ties with her family and friends. She has her green card and her prince but is otherwise quite alone.
Note: Quote for 8a and 8b is from Princess and the Frog.