This morning I made an addition to my semi-arid print-out battle mat, a strip of coastline! It's an overlay about one and a third A4 sheets wide, by three A4 sheets long.
Basically, I loaded up my map image in a graphics program, and placed an enlarged top-down coastline photo on top of it, then used a fading/erasing tool to remove the land from the coastline and blend the water into my own terrain image. Then I cropped just the end of the map (where the coastline was) and saved it as a new image.
Printing it out, it can be laid on top of the end of the map, to create a seamless coastline addition! Or it could be used as the shoreline of a lake or wide river. I might make a second backdrop, so that if I place the water against my wall-end of the table, the backdrop has water leading off to the horizon with some blue sky and clouds, for an endless-ocean look.
Showing how it just lays on top of the map: