Maybe take a look at Eneroth Cut Surface Organizer.

You can export the alpha of the render and replace the HDRi with a beach image. The alpha is the black and white channel which you can use to tweak or replace what's behind transparent surfaces. In Frame Buffer window you have a white circle near RGB values, switch that on and save your file. Then in Photoshop or Gimp or whatever post production software you use, replace the black with your desired image of a beach.