This tutorial will teach you how to take two layers (one with sky on it, the other with water), and make them look like they can from the same photograph.
First you'll need to download the example file;
this is in Adobe Photoshop format. If you intend to follow this
tutorial with another image editing software, you can download this layered tif file.
Step two - Now we need to change the contrast and brightness, so go to image>adjustments>brightness/contrast. We're going to need to increase the contrast so the overall water color gets darker, but keeps the highlights so as not to make it look flat. We'll also increase the brightness so the image doesn't get too dark. The settings are in the image below.
Step three - We need to flatten this image so it's just one solid layer. To do this go to layer>flatten image.
Step five - Now go to the refine edge... on the tool options bar underneath the Adobe Photoshop menu bar (this button is underlined in blue on the image above). We need to make this a softer selection, so when we apply the filter to smooth the horizon it will gradually fall off. Set the settings to the image below.
Step six - Now go to filter>blur>Gaussian blur. Set the radius to 2.0.
This technique can be used for many different situations. Of course you'll have to change some of the settings above, but the work-flow will basically be the same.