I meant to update and let you guys know how I went...

We all went op shopping, and found some very cool pieces to use. The first was a little jellyfish toy. It was a little beanie filled thing, with long tentacles, and a lovely rainbow fabric. I sewed some hat elastic that I had at home to it's underside (hooray for hoarding), and dd wore it as a little hat. Second was a very open knitted poncho, in a dark ocean blue/teal. It was a little on the big side, but it draped over her a bit like wavy water. Not bad buys for under $4 total.

Topped that off with about six strands of blue, purple and clear crystal beads tied into long necklaces, from the cheap shop.

From memory, she wore a dark blue long sleeve tee shirt, but I can't remember what pants or skirt she wore.

It was good however, as all but one of the other girls were dressed as mermaids.