Don’t mean to res a one month old thread, but I’ve read that you need to enable edit layers on your landscape BEFORE you add the landscape. Meaning, no landscape should be in the map, click on Modes, select Landscape, BEFORE adding the new landscape, select the checkbox for Enable Edit Layers, and then ADD the landscape. Then add your water body. The problem with this method so far is that it just plain sucks for people wanting to add the new water systems to their existing maps.