Since I am not using the methods presented in Part 6, our file still consists of just two layers: BG (the bottom background layer of sky and water) and Andrelica (the top layer with no mask). Let's submerge her in some water. Make sure the top layer with the model is active. Hit Command-R to show Photoshop's rulers. From the top ruler, pull down a guide to approximately where you want the waterline to be. Remember that Snap had been turned on to help position the sky image. It must now be toggled off, View -> Snap. If we do not do this, the waterline we will be drawing will snap to the guide, and we do not want that.
The key to drawing a waterline is to remember that parts of the model further away from the camera will have a higher waterline. Those parts closer will have a lower waterline. Activate the Pen tool (P). You may want to zoom in a little on the guide and the part of her lower than the guide. We are simply going to create a path by using the Pen tool in a "connect the dots" fashion. Start by placing a point out in the water above the guide, then go across the model, guessing where the waterline would be. Once you have gone across the model, click out into the water, and go around her all the way to the beginning dot. Click on the first dot to close the path. It should look something like this:
Hit Command-Return to convert the path to a selection. Hit Command-J to put the selection on its own layer. Name this new layer Underwater. Get rid of the guide View -> Clear Guides. At this point, the Layers Panel should look like this:
Next we want to modify the Underwater layer to make it look under the water. Before we can do that, we must mask out the under water part on the Andrelica layer. Add a mask to the middle layer of the model. Command-click on the layer thumbnail of the Underwater layer to get the marching ants. Notice that the marching ants do not include area where there is water - they snap to her. Fill the selected area with black. Since black is the foreground color, that would be Option-Delete (Control-Backspace). Command-D to deselect the marching ants. Turn off the visibility of the Underwater layer by clicking on its eyeball in the Layers Panel.
You will probably see an undesirable light outline. Paint over it on the mask with a black Brush. If your Brush doesn't seem to work, it is because it is still set to Overlay, from when we used it to "firm up" the edges of the background mask. Set it back to Normal in the Options Bar.
Water absorbs light, so we want to darken the Underwater layer a bit. Add a Curves adjustment layer about the Underwater layer, hit the clipping button (the icon on the lower left side of the Curves Panel) so that the darkening will only apply to the Underwater layer. Pull down the midpoint of the curve, and you should see something like this:
Water also blurs, so activate the Underwater layer, and go to Filter -> Blur -> Gaussian Blur... and run a blur. I used a Radius of 13.4 pixels. Use less if you have a smaller file:
The next task is to fade the underwater part. We should be able to see less of the model where she is deeper. Add a layer mask onto the Underwater layer. With black as the foreground color, hit G to activate the Gradient tool. With the linear gradient selected in the Options bar (the first of the five icons), run a black to transparent gradient on the mask by Shift-drgging from the lowest part of the model to highest portion of the waterline:
This has the effect of fading the "deeper" parts of the model. Next, we want to fade the entire Underwater layer a little. Do this by lowering the opacity of the Underwater layer. I lowered it to 40%:
I suggest lowering the opacity of the Underwater layer to show a little less than you want. This is because the next step, where we add a reflection of the model in Part 8, will bring some of the opacity of the Underwater layer back.