I saw something similar in a cake book and wanted to make my own version of a Little Mermaid cake, so here is it. I hope you like it!
For the board, I cut chunks of board away from the edges, so the board wouldn't be a perfect circle, and then covered it with a thin piece of fondant, then set it aside to dry. I painted the fondant yellow, let it dry again, and then painted it a copper/brown color to make it look more like the sea bottom.
I used fondant, with different food color gels added, to mold the fish, coral, vines, star-fish, and mermaid, by hand.
For the mermaid (copper color gel), begin with the body and tail (no fins yet) as one piece. Use a size 8 round tip to make indents in the tail section of the mermaid to resemble scales, then set it aside to dry (Don't forget to position her tail and body as you would like it to appear on the cake). Hand-mold the fins (I used the flat side of a knife to make indents on the fins to give it dimension), arms, head, and bodice shells (again, I used the flat side of a knife to give dimension). Set the pieces aside to dry.
When the body (tail section only, bodice shells, and fins are completely dry, paint them with blue food color gel. When the body is once again completely dry, add some blue crystal flakes to a little bit of water, then paint the lower half of the body (tail area).
I baked a 4x2, 6x2, and an 8x2 inch cakes (pound cake works best), then cut away the top and bottom edges to make them rounded. I added blue food color gel to fondant, kneaded it until it was completed blended, and then rolled it into a log shape. Once in the log shape, I dipped a tooth pick in moss green food color gel, and then poked the fondant log in several places. I blended the fondant slightly (you want to keep the marbling effect). Cover the cakes with butter cream icing, then with the marbled fondant.
Use royal icing to glue the fish, coral, vines, star-fish, and mermaid's body to the cake, then attach the mermaid's arms, fins and head.
Use brown fondant to make hair for mermaid, then before the hair dries insert stamens to make a crown. Use food color gel and a very small paint brush to paint the mermaid's face.
Note: It took me three attempts at making the mermaid's body before I was satisfied with how it looked, so don't give up.