A fair test for determining how proficient you are in creating realistic-looking water images with GIMP is comparing your invented water texture with the real thing. You can create an artistic approximation of this image as painters in pre-digital eras did--with just pigment and paintbrush. But if you want to fool viewers into thinking they're looking at water, learn to apply GIMP's filtering tools for manipulating images algorithmically. Using GIMP's filters to create a water texture involves setting parameters of the "Wave," "Blur," "Gaussian" and other filters.
Click the "Filter" menu, and then on the "Render" submenu, click "Clouds." Click "Solid noise." GIMP opens a dialogue box for you to enter parameters for the background element of the water.
Type "2.3" in the "X" text box, and then click "OK." This action creates a turbulent background on the canvas.
Click the "Filter" menu again, and then click "Blur," followed by "Motion blur."
Click "Radial" in the dialogue box that appears, and then set the "Angle" control to 50 degrees. Click "OK" to tell GIMP to blur the background you created in the previous step.
Click the "Filters" menu's "Blur" item, and then click "Gaussian," which is another kind of blur. Type "37" for both the "Horizontal" and "Vertical" text boxes, and then click "OK" to create the blur. The image is not yet recognisable as water. It's only a background on which you'll create the water texture by using the "Waves" filter.
Click the "Filter" menu, and then on the "Distorts" submenu, choose the "Waves" filter. GIMP opens a dialogue box with controls that you can set to impose a waterlike wave on the background of the preceding steps.
Type "50" in the "Amplitude" textbox, and then type "185" in the "Phase" text box. Type "35" in the "Wavelength" text box, and then click "OK" to tell GIMP to create the wave.
Right-click the "Background" item in the "Layers" palette, and then click "Duplicate" to make a copy of the "Background" layer. This layer will multiply the water effect of that layer.
Click the "Layers" palette's "Mode" drop-down box, and then click "Difference." This tells GIMP to mix the two layers by subtracting their pixels' colour values, which results in patterns not easily achieved through other tools.
Press "Shift" and "R" simultaneously to run the "Rotate" tool. Type "175" for the "Angle" in the dialogue box that appears, and then click "Rotate" to perform the rotation and complete the water texture.