I had a moment of inspiration to make this the other day. From the start I knew I wanted the background to match the darkest color in the picture of the ocean, so that it gave the impression that they blended. I also knew I wanted to have foamy mist pouring out of the box.
So, what I did was find a picture of the ocean (top down) and used a mask to make it square and centered. I adjusted the curves to try and darken the dark places and highlight the foam. Then I duplicated that layer, scaled it from the bottom left of the centered square to the top right of the image and began adjusting its own mask.
I tried painting the alpha channels, but that didn't leave any mist. It also looked uber fake and boring. Instead what I did was duplicate the image, then I threw it into the mask layer and fiddled with the overlay properties (all these years and I still don't know what that thing is called) until I found one that made all the dark areas transparent but left the foam and mist to appear as if it was being slung out of the picture.
