Marble Texture

Creating marble is a useful trick to know. The final image can also be set to grain merge over solid blue to create a water texture, an example of the water will be shown at the end

I like to start my textures at 512x515, you can always shrink it later if you need it. Starting with a blank canvas layer click Filters>Render>Clouds>Solid Noise. Adjust your settings to the ones below.

You should end up with something like this:

Next duplicate the layer, invert the colors, clicking Colors>Invert and change the layer mode to Difference. Merge the two layers together.

Next click Colors>Levels adjust your settings to the ones pictured.

You should now have something that resembles the image below.

Finally, duplicate the layer and click Layers>Transform>Rotate 90° clockwise. Set the layer mode to screen and adjust the layer transparency to whatever pleases you and merge down. That's all you need to do!

Here is the water texture:

P.S. I have included pictures in good faith to help you on your creating journey. Please do not steal my textures, creating your own is much more satisfying. Stealing my texture will result in a DMCA complaint lodged against you and I would hate for it to come to that.