I was tempted reading the comments about Merida not being level-headed enough, but I think that being somewhat connected to nature gives her the edge here. And Elsa does have her ice powers

[UPDATE] Given to the points people made about Merida, and to solve the tie, I'm changing my own pick to Merida.

I feel like she'd be horrible at it because much like Maui, she relies too much on physical violence and brute force to get things done. When she was firing arrows at Mor'du, it's pretty clear that her firing arrows isn't working AT ALL but she continues to do that and run away instead of trying to find another way around it. Moana was chosen because she is physically capable of fighting but was also compassionate and didn't rely on brute force too much. That's why the Ocean never had Maui be the one to return the heart. He would've relied on fighting and couldn't subdue Te Kā with compassion and kindness long enough to restore the heart and return Te Fiti to her true self. The same would happen with Merida. She's all action (Which she's not even very good at) and too impulsive and isn't compassionate, patient, or kind enough. I'd say that only Aurora would've been ARGUABLY a worse choice than Merida.

Cinderella has proven that she is resourceful and physically capable in the third movie. Plus, she has a kind and compassionate heart, as well as A LOT of patience, so Cinderella could actually succeed, though she wouldn't be one of the best choices.

She doesn't have much life experience outside the house.

Agree with KataraLover, she'd be too focused on punching her way out of the situation. My next choice would be Cindy because, well, I can't imagine her surviving the ocean.

Same as before. She spent all of her movie running away from problems, and she's really skittish about everything.... So I don't see her saving the day.

Agree with wavesurf.

Merida is strong and all but she doesn't think things through too much