I FOUND A WAY INTO WATERMILL WHEWT WHEWT!
Here's a StepBy Step thing on how I accidentally got there.
It will require at least two computers both with FH installed.
1. Take your char to 'Water'Mill'. It doesn't matter how it looks, just do it.
2. Log-Ogg leaving your char there.
3. Log on on the other computer with the Char you left in water-mill.
Now, this will only work if you didn't touch the Water-Mill map on one of the computers, other-wise you will not see it.
It might have a portal, I couldn't find one(Then again, it's a maze of a map) And if you don't want it to be there forever, you may want to re-set home or just go home.