I may have randomly had the sea witch dress problem solved. Sam came home with a dress from work that someone didn't want...that was in my size...and is turquoise/sea green...and has a flouncy skirt...and looks halfway decent on me. I'm suspicious of this too-good-to-be-true item that has dropped in my lap, and so am cautiously optimistic. I think I can definitely alter it a bit to make it work for a sea witch costume.
Here's a couple of pictures of the dress--it's extremely flouncy, and the upside is that I would eat no guiltburgers in cutting it up and having my way with it.