My plan was to use all thrifted materials (of which I have currently found 90% of everything I need and spent about $5) to build the costume.
First, I'd start with an old gown or prom dress in blue - hopefully something with a full skirt that would simulate the 'sea.'
Then, I'd use a black velvet dress that I'd bought to create the body and tentacles of the octopus. The 'body' would attach to the bodice of the dress for structural support and the tentacles would be stuffed with polyfill so that some could curl, some would hang, etc. I also envisioned that the tentacles on the side would attach to my wrists, so that there would be some 'movement' to the costume.
Under the dress, I'd wear a purple long sleeve shirt (for warmth and also to avoid body paint), then make the golden shell necklace and the wig and viola! (Plus, add some fish to the skirt to underscore the fact that it's supposed to be water)