One of the most valuable things we can do with desktop versions of Photoshop is make precise selections in order to focus our edits to a very specific region of the photo. We can do this by leaving the selection active and editing, creating a duplicate layer with a layer mask based on the selection, or we can create layers with transparency (also based on the selection).
With Photoshop Touch for iPad, we still have the ability to create new layers with transparent areas, based on a selection. This means we can isolate our edits to a very specific area, and not impact the original background or base layer, even on iPad! Here we'll select the water in order to create a special layer for same.
For this example, I used the Lasso tool, which looks just like it does in Photoshop CS6, and is found in the Toolbar, on the left side of the screen. Just tap it with your finger or stylus to select it.
When you do, the toolbar will change to display the active tool, and whatever options are available for it. Here we can see the familiar add/subtract option and the anti-alias option.
In Photoshop Touch, we can use the Lasso tool and our finger (I recommend purchasing the Bamboo Stylus from Wacom for this type of function) to draw a rough edge inside or around our subject. It doesn't need to be 100% precise at this stage.
When you're finished you should see marching ants identifying the pixels you've selected, just as you would in the desktop versions of Photoshop.
Next, from the Selection menu (top of the screen, fourth icon from the left), choose Refine Edge.
This will display a QuickMask view of the selection you just made (reddish color), along with some options at the bottom of the screen. Use the default Brush mode if you wish to expand the selection, or the Eraser mode if you wish to reduce its scope. You can also modify the cursor diameter by clicking on the Size icon (white circle) and swiping to the left when the brush preview appears.
Once you're ready, place the cursor on the edge of your original selection and use the stylus to brush over the boundary area (in this case everything touching the water) until it's entirely revealed. The shot below shows this step nearly complete.
Once you pick up the stylus, Photoshop Touch will process the selection for a moment and then create a much more accurate boundary shape. There may be a few spots that need further tweaking. Go ahead and click OK.
The result should be a new selection with marching ants, that is more accurate than before.
Sometimes Photoshop Touch will mistakenly add small areas of new pixels or omit areas you intended to select. You can typically use the Lasso tool again to quickly (using the add / subtract modes) to remove or reincorporate small regions of pixels, just as you would in Photoshop CS6.
Once your selection is done, click the plus (+) button at the bottom right of the Layers panel (right side of the screen). With the selection active choose Layer from Selection.
This will generate a new layer that includes only the water pixels and shows the rest of the document area as transparent pixels (meaning what's underneath will show through).
Now you're ready to perform your retouching or edits on the new layer! Here I darkened the water by change the Blend Mode and Opacity.