There is a very small area at the edge of land where the top of the water plane is visible but the land can still be built upon and perhaps routed through (use "Show Routing" to be sure).  If you can catch this edge through trial and error and use "Flatten terrain" to extend it, you can build on the water.  Land will be visible right under the surface, but can be disguised with a water-matching terrain paint and plants, or by strategically lowering parts of the land after your building is complete.