Every mega-dungeon needs an underground sea area, right? Found some fairly inexpensive foamcore with a nice green base color. For this type of terrain brown or blue would have worked as well. Just slapped some spackling compound on the edge portion for a shore line. I love spackle because you can really pile it up if need be and it comes in great big tubs if you need it.
Painted some 50/50 white glue and water randomly onto the shore areas to give it a more interesting texture.
Next came some random black spray paint.
Finally, after 3 coats of clear gloss finish (warning, it's oil based, just about guaranteed to wreck whichever brush you use on it), I have completed the shore line of my underground sea. If my players figure out some way to get/find a boat/raft down there and want to sail, I can easily whip up more a couple more for that endless seas appeal!