I made this Under Sea Birthday Cake for a dear friend whose daughter was turning 5. This was a very small cake, 2-6 inch rounds with blue butter cream. I put it on a pedestal plate because there was a Little Mermaid doll next to the cake and I wanted height.
I used marshmallow fondant to make the details. The starfish I made starting with a ball of fondant and pinched out the arms and flattened and molded to make them look right. I then pressed in small holes with a sharpened dowel down each arm and scored a star in the middle with a knife. The yellow sea sponges are again balls of fondant and I pressed an eraser into them for the hole. The pink coral is fondant rolled into long snakes and cut into different sizes for height variance.
The seaweed is actually a leaf tip that I used medium pressure and squiggled various heights and overlapped as well. Flounder was mainly yellow fondant starting with a large ball and added layers of blue where necessary and a small yellow nose. I painted with food coloring the dark blue and black for him and the angelfish as well. I use a 2D tip to make the waves to help Flounder up out of the water to see Ariel. Very small candy rocks and a sea horse chocolate mold finished the look.