There's one effect that I'm quite fond of, if I do say so myself:

The edge effect is produced by the following steps:

1. Start with a white background. This is your water.
2. Paint the landmass in black.
3. Use the wand select to select the landmass and copy it to a new layer, called 'land mask'. It will usually be invisible, but is extremely handy to refer to when applying effects you don't want bleeding into the water.
4. Do the same for the water, and call it 'water mask'.
5. With the original white and black layer, use detect edges and set the line thickness to 2 px. (or more, if you like). Call this layer 'coastline'.
6. Duplicate land mask and paint it in your land colours as you like.
7. Duplicate water mask and paint it in your water colours as you like.
8. Duplicate coastline and place the layer above the land and water masks at somewhere around 50% transparency.

The result is an edge effect that is half water colour and half land colour, nicely outlining your landmasses without being as obtrusive as a solid colour.

Yes, I like to keep a lot of layers full of useful phases of the image's construction, even if those layers end up invisible.