In the Photoshop Tutorial: Create Water Reflection for Architectural Illustration you can follow in detail how to create a reflecting water surface in Photoshop quickly. For many people, this is surely a valid alternative to directly rendering the water surface, especially when it needs to be done fast and you’re not familiar with the render settings. The core of the process is creating a displacement map with which you can create the effect of a water surface on a mirrored part of the image.
We are giving the displacement map away as a free download under this link:
This is a Photoshop file which also has both layers from the creation process.
Therefore you can change the settings in order to experiment with different variations of the displacement map.