It was done quickly in Gimp using the following steps.
The essential pieces are three layers (1) a colour layer for all the blues and greens (2) an overlay layer to get the light and dark of the water and (3) a highlights layer to get the light reflecting off the top of the water.
The Colour Layer
1. Create three layers.
2. Fill the bottom layer with a nice blue-green-grey.
3. Fill the next layer up with a greener shade
4. Add a layer mask to this layer. (layers dialogue, right click on layer->Add layer mask)
5. Click on the layer mask (square to the right of your layer in the layer dialogue) so it is outlined in white.
6. Go to Filters->Render->Clouds->Plasma
7. I generally stick the turbulence quite high to get a lot of different patterns in the plasma.
8. Now you should have some bits of your green showing over the blue below. Play a bit with the layer mask - applying a blur to it is usually a good idea - until the two look nice and mottled.
9. Repeat steps 3-8 with another layer and different shades until you get a mottled blue green effect NOTE: Remember to pick a new seed in the Plasma dialogue each time or all your colour will have the same mask.
10. Merge all these layers together. This is now the base water colour.
You should now have something like this:
The Overlay layer
1. Create a new transparent layer.
2. Go to Filters->Render->Clouds->Solid Noise
3. Check the box marked turbulent
4. Play with the x-size and y-size until you have a nice detailed pattern in the preview window then press okay.
5. Go to Layer->Colours->Invert
6. Make sure this layer is immediately above your water colour layer (move it using your layers dialogue).
7. Change the layer mode to overlay.
You should now have something that looks like this:
The highlights layer
1. Duplicate your overlay layer.
2. Change this new layer's mode back to normal for the time being.
3. Go to Layer->Colours->Threshold.
4. Take the black marker on the histogram to 1.
5. Move the white marker down slightly (around 245 usually gives a good result) and then click okay when you are happy with the preview.
6. Invert the colours of the layer again so now the thin lines are white.
7. Change the mode of the layer to lighten only.
8. Reduce the opacity of the layer until it looks good.
You should end up with something like this:
Play with the opacity of the different layers until you are happy. Remember you can also change the colours afterwards. In the example above the water shades are very bright, so I decided to lower the saturation and darken the whole thing down a bit to get this:
Hope that's of use to someone. I enjoyed figuring out how to get Gimp to do this. If people have any questions about this, or suggestions about how to do this quicker, please let me know.
