RobA - thanks a lot, that's a brilliant tip. Yep, that's definitely going to change the way I use Gimp.

Now the map posted above is a bit flat. Yes, it is clear and every element is distinct, but it's not particularly inspiring. To make the image pop we are going to add highlights and shadows. To do this we use an overlay layer.

First we will do general light and shade. Create a new layer and fill it with 50% grey (#808080). Your whole image will go grey! Don't worry, set the layer blend mode to overlay. Suddenly yourwhole image will go back to normal. Name this grey layer 'Sea light/shade' or somesuch.

Now The purpose of this layer is to give light and shade to the whole piece. This is done through overlay. Any colour value above 50% grey will have the effect of lightening every layer beneath it - up to and including your parchment background. The amount it lightens the layer is proportional to how much lighter than 50% grey the value on the overlay layer is. The opposite is true for values darker than 50% grey.

So to block in some light areas we need to lighten the overlay layer away from 50% grey. Now we could do this by laying down a lighter grey with the paint tools, but I find it easier to usethe dodge/burn tool on the grey layer. There's a good reason for this, but I won't go into it here unless people are interested.

Go to the dodge/burn tool and pick settings such as these:
Click image for larger version.  Name: dodgeBurn.jpg  Views: 135  Size: 38.6 KB  ID: 4433

Now, dodge lightens tones whereas burn darkens them (it's a little more complex than that when dealing with colours but the purpose of what we are doing here this will suffice for now). By using a large fuzzy dodge brush on your overlay layer you will see whole areas of your map lighten up. Switch to burn to drop in the shadows. So for example if we have an overlay layer that looks like this:

Click image for larger version.  Name: overlayLayer.jpg  Views: 197  Size: 40.1 KB  ID: 4434

and we apply this overlay layer to the image I posted above, then we get something that looks like this:

Click image for larger version.  Name: withOverlay1.jpg  Views: 219  Size: 123.6 KB  ID: 4435

Now obviously a lot of the overlay regions that are making the waves dark are also darkening the ship, which is exactly what we don't want. Do we go back and paint those bits of the overlay 50% grey again? No! We just add a layer mask to out overlay layer. Copy and paste the layer mask from the sea colours layer into this new layer maks and the image cleans up immediately:

Click image for larger version.  Name: withOverlay2.jpg  Views: 245  Size: 124.8 KB  ID: 4436

See, I told you creating those layer masks would be useful in the long run!