We have the piece of the puzzle on a new layer but we have to hide it on our beach layer otherwise we won’t be able to achieve the desired effect. Click on the “beach” layer to select it and click on the layer mask to make it active. Now click Ctrl + Backspace to fill the mask with the background color (it should be black, but if it’s not press D to reset colors to their defaults).
Nothing seems to happen because although we hidden the piece of the puzzle on our beach layer the “puzzle1” layer it’s still visible. If you turn off the visibility of the “puzzle1” layer (by clicking on the little eyeball icon to the left of the layer icon) you will see that our masking worked. Turn on the visibility of the “puzzle1” layer.