Heres a tutorial for making birds eye views of continents.
Starting
First, make a medium-large sized image (I did 1000 x 1000).
Make a new layer, and render plasma clouds with a complexity of 7 (max).
Make 2 new layers. Render solid noise with an X & Y size of 16, detail set to the max. Do the same to the next layer, but use a different seed. Set the top solid noise layer to difference. You should now have what looks like solid noise with little black lines running through.
Merge the two solid noise layers, and invert the colors.
Go back to your plasma clouds layer. Desaturate it, and bump map it to the SN (solid noise) layer with a depth of 30
Gaussian blur the PC (plasma clouds) layer for 15 on X & Y. Then, bump map it again to the SN layer with a depth of 30. This will be the basis for your continents.
You no longer need the SN layer.
Continents
Make a new layer and fill it with black. It makes your upcoming selection easier to see.
Use the lasso tool and make a completely random selection on the black layer. Make as many seperate shapes as you want, and be sure to include other smaller selections to be islands. To go the extra step, make each selection look like it could have been removed from the other, as though they shifted apart. To add lakes and other interior bodies of water, invert the selection and add blobs to the inside of the continents. Re-invert the final selection. Here's an example:
(note that yours should be a lot more complex then this. I just did this one quickly)
With that selection, go back to your continent basis (the bumpmapped one), invert the selection, and clear everything around it. You don't need the selection any more, nor do you need the black layer.
Coloring the Continent
Make a new layer. Choose a gradient (I used "greens") and put it on the layer. Then, set the gradient mode to "soft light" and drag it across once or twice more, in different directions. Set the gradient layer to soft light, and make both layers visible. Looks good huh?
Now, go to the continent layer and alpha to selection. Go back to each gradient layer, invert the selection, and cut/paste the remaining onto a new layer. This will come into play for the water.
You should now have three layers: The continent, one gradient in the shape of the continent, and one gradient in the shape of everything that
isn't the continent.
Make all layers invisible.
Water
Start off by rendering plasma clouds with a complexity of 4. Be sure to use a different seed then the land. Blur the PC layer at 40. You don't want to many black or white areas. Use the brightness/contrast tool and tone down the contrast, or fiddle with the levels or curves if you know how to use them. Then, colorize it (I used Hue: 205 Sat: 45 Lightness: 5). Create another SN bumpmap layer, and map it to your water layer with a depth of 15. Gaussian blur the water at 30. Remember the gradient layer we haven't used yet? Move it right above the water layer, set the mode to value, and the opacity to 50. After this step, you may want to lighten the water layer a bit.
Now, make all layers visible. One last step: Select the water basis (PC) layer, and bump map it to the continent basis layer. Set the depth to 65, and the elevation to 60. This is simply to seperate the land from the water a bit.
Finalizing
Go to the continent basis layer, and alpha to selection.Grow the selection by 8 pixels, and (on a new layer) fill it with white. Gaussian blur the white by 15. Now, alpha to selection on the continent layer again, go back to the white layer, and clear. You should have a white ring a bit bigger then the shape of the continent. Move that layer right below the continent layer, and set it to soft light.
Your done!
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