"Message in a bottle flowing on waves. Fake underwater effect".

 

• Select top white background wit Magic Wand Tool and remove it.

• Move water wave little bit down, like on the image below. Erase bottom edge with soft Eraser Tool.

• We need to remove top part of the underwater layer(red marked area on the image above).
• Find underwater layer, add layer mask to it, load selection for the water wave layer, invert it - Select > Inverse. Process selected area above water wave in the Layer Mask with black brush.

• Make the same thing with the underwater layer clipping mask above island layer:

• Return to the inserted wave layer. Duplicate this layer two times. Make duplicates invisible, we will use it in the future steps.
• Add Layer Mask to the first, bottom wave layer. Fill bottom part of the wave with black&white gradient in the Layer Mask(!). Then Erase top Edge with smooth Brush Tool in the layer mask too.

• You should have something like this:

• Apply Multiply Blending Mode to this layer.

• Create "Levels" adjustments clipping Mask to created layer.

• Apply following settings:

• Make visible second copy of water wave. Go to Image > Adjustments > Invert. Then Image > Adjustments > Auto Contrast. Erase top part of this layer like on the screen shot.

• Apply Screen Blending Mode to this layer.