This time, we go Under the Sea in Atlantica.
I don't have much to say about this level, surprisingly enough.
There some interesting parallels between Ariel's desire to explore other worlds and the same desire that started Sora, Riku, and Kairi's original journey. Even better, the writers saw fit to highlight it and bring it into focus. So while nothing here directly advances the overall plot, it does tie into it on a thematic level.
Shame Kingdom Hearts 2 really pissed all that away...