Obviously water. But also, a lot of birds seemed to like her (they were helping her hook up with Eric- why else would birds do that?) so maybe water and flying.

If Misty doesn't become the world's best water pokémon master then Ariel will. Definitely anything with one of its type as water, though I could also see her using ice types...whic is basically just frozen water. XD

Duuuuhhhh
I also see ice and flying like you said.
She'd probably have all like "cute" pokemon in her party like Merils, horseas, and goldeen.
Basically she'd be Misty XD