Ok, go into map maker, load WaterMill, press export, make an object list for all your objects, and that SHOULD work. If it doesn't, let me know, and I'll experiment with it myself and figure it out.
EDIT: @Ruby and Hammy the .fhz won't work, and you're going to have to export it because of the new patch...plus the object list because of the new patch as well.
And yes, you have to do the portal as well, to place it in CapeOfWorlds