if you still have your project folder..
Open TheShadows Folder ( your map folder in Media>Terrains)
copy/ paste watermillMask.png and watermillTerrain.png
rename the copies "TheShadowsMask.png" and "TheShadowsTerrain.png"
do NOT delete the old "watermil" images yet.
Load FeralHeart's Map-Maker, and import the renamed Terrain and Terrain-Mask where they belong.
Export and test.
If this doesn't work - send me via pm your map project folder.
also Double-check to see if you have "Has Portal" checked.