Mermaid costume help!?

I am going to be Ariel, the little mermaid, for halloween this year. Does anyone have ideas for a cute costume? I'm a teenager so I'd need an adult or large child's size

temporary hair dye? make up?

I was thinking this for the purple shells instead of fake shells....http://store.americanapparel.net/rsa8341.html#i

that and wear a long skirt that looks like Ariel's one and wear aqua colored flip flops. Why won't you put a flower in your hair and add some glitter to your face?