Take a piece of green-ish paper. Cut 2 tear shapes and a triangle. Glue the two teardrops' fat ends to the point of one of the triangle sides. For a 3-d affect use two traingles together, you can use glue or tape.
Do you have a bathing suit you are using as a basis? You could trace the design of a tail on a piece of fabric that you have doubled. It should fit from the bottom of he bathing suit, to the ground for length. Then sew the side edges together and attach to the top part of the tail to the bottom of the bathing suit. I would make it so the bottom of the tail is open so that you can walk and kick your legs.