You can create some fairly nice water/shore lines in Carrara as is -- though I'm sure Eric's plugin will make life much easier.

Add a Terrain shader layer to your terrain, then select custom distribution and set the shore line color band with altitude, slope, and any other available options you want to use, to define how high the shore line will be relative to the water. You can add multiple layers -- just experiment.

Then go to the terrain editor and add a noise filter layer, which has an altitude/slope that roughly matches the shore line color band that you created. Play with noise size and amplitude (and other settings) until you get a rough noise area that matches the shoreline color band.  This will break-up the water that touches the shore, and will make for complex light interactions that make the image a bit more realistic.  Use GI, Sky Light to render.

The attached image has very shallow pot holes filled with very salty water.  I've done other shore lines with this technique, and it seems to work fairly well.  Still, I'm looking forward to Eric's new plugin! :-)