Mix blue food coloring into some fluffy buttercream to get a light aqua color. Then scoop a tablespoon of the buttercream into a piping bag. Add less coloring to your next tablespoon of buttercream and continue the process until you only put white buttercream in. Make ruffles by starting at the top of the cake and steadily squeezing the frosting through the piping bag. Make sure to keep turning your cake to create that soft wave effect. Keep going until you reach the bottom of the cake. Voila, you have waves.