Prepare cupcake pans by lining them with cupcake liners. Set aside.
In a large mixing bowl, combine flour, sugar, baking powder, and salt. Set aside.
In another large bowl, beat milk, vegetable oil, vanilla, and eggs together.
Add dry ingredients to wet ingredients. Beat again until smooth.
Add water and beat again.
Transfer batter to prepared cupcake pans, filing each liner ⅔ full.
Bake for 15 minutes or until a toothpick comes out clean. Allow cupcakes to cool completely before frosting.
To prepare frosting, in a medium mixing bowl, cream butter and powdered sugar together.
Add 2 tablespoons of milk and vanilla. Beat well.
If it is too thick, add more milk. If it is too thin, add more powdered sugar.
Next, add food coloring gel until desired color is reached. This will vary depending on the type of food gel you are using.
Transfer frosting to a large ziplock bag and cut a small corner off.
Pipe frosting onto each cupcake, adding sprinkles before the frosting hardens.
Carefully place a mermaid tale on top of each cupcake.