Well, I finally figured it out: how to make shores look "wet". Let's face it--when waves are breaking on the shore, the waterline isn't completely dry as appears by default in terragen.
So here's what you do:
1. Create a fractal terrain for yourself, add a lake, and find a camera spot near a shore somewhere.
2. Create whatever surface layers you want. Sand, rock, moss, etc. I added fake stones too, just for effect.
3. Now create a surface layer and call it "wetness". Uncheck the "Apply color" box under the color tab. We don't need the color. The coverage should be 1. We don't want any places to be dry that shouldn't be.
4. Go into the altitude constraints tab and set the maximum altitude to whatever your water level is at. Set the fuzzy zone to about 0.5m (50 centimeters). Use less for calmer waters. You could also set the max altitude a little higher than the lake, if you wish, for very rough waters. Also, you'll need to check the "Use Y for Altitude" box.
5. Now click the Add child layer on the "Wetness" layer item in the list. Go to "color shader" and add a color adjust shader. Click the Gamma tab in the color adjust layer and set the gamma to 0.5. We want the color of the wet parts to be 50% darker than the normal colors of the shoreline. Water darkens the surfaces it wets. You can use more or less darkness for this part if you wish. Tweak it to whatever looks right in your scene.
6. Click the Add child layer button on the wetness item again, and go to "Other Surface Shader". Add a reflective shader as a child of "wetness". You might want to adjust the reflectivity and intensity to your scenes needs. Higher levels make the shine look sort of plastic.
You're done. Render away. Now your shores don't look so artificial. I exaggerated the levels of wetness in the example below just to show it. With such calm waters the rocks would not be wet so high up.
PS. You can also use this "wetness" layer with small amounts of coverage in a non-shore scene to make a rain-splattered effect. Use a very cloudy sky of course.