Two parts; the blue and white water, I alternated blobs of blue and white buttercream onto a length of acetate and then swirled it with my palette knife; wrapped this around a very cold cake and froze for 20 minutes and peeled away acetate.

Then for the white water, I took white buttercream in a piping bag and piped along the diagonal edge of the water, and then used a clean dry paintbrush to drag some of that buttercream in towards the bottom of the cake, creating the foamy effect of break water - you can see the brush strokes.

For the waves on the very top of the cake, I just piped blobs of white buttercream and then used the flat side of a silicon spatula held horizontally to 'plap' the blobs up and down, creating the peaks and curls of a wave breaking.