My husband and I went as a mermaid and fishermen duo. For this fisherman and homemade mermaid costume idea we found almost everything at Value-village and other second hand stores.
For the top I used one of those hold-you-in-full-body-bra-things you can find at Sears or the Bay. Than I used some silk fabric found in the sari sections of second hand stores. You could use any shear fabric that is in different shades that work well together. I used greens and blues. I tied and safety-pined the fabric around me like a skirt as tight as I could. The hard part was making sure I put it on so that I could just take a few of the safety-pins out to get it down to go to the bathroom.
I used seaweed from my sisters old aquarium, and pined it on here and there. I was making a slight, dark, joke with the filth floating in our Oceans and put some plastic bags and pop/beer rings on me. It was sort of a washed up mermaid idea. I have my lip pierced so it was easy to gross everyone out by getting a barb-less fishing hook and sticking it through my hole. I tied some line to it for effect.
I found the wig at Walmart. I had to tie a strip of fabric around like a head band to hold the wig on better.
You can't tell from the photos so much, but I added some blue face paint to a cheap cover-up and used it on my skin for effect. I also used green on my lips and eyes. It worked really well. I found some green shoes that were nice and small.
My husband got a dark-blue, heavy sweater from the second hand store, and a fishing hat from the fishing supply store. He carried a fishing rod around for part of the night and had some plastic fish strung up together. They were from a set of lights found at Canadian Tire, I think.