My 7 year old daughter wanted a homemade mermaid birthday cake for her birthday. To make the structure of the cake, I divided the mix between 2 sandwich cake tins and a few muffin tins (for the rocks). I got the sea effect by putting in blue paste food colouring into the white fondant, and mixed it together just till I got the marble effect I wanted and then rolled it.
Finally for the mermaids, I used the fizz and surprise mermaids my girls have been collecting, and stuck them on to the cake using icing. My girls loved it.