Wisty Willow is a pocket dimension,it appears to be a small deserted island in an infinite ocean. Strangely this pocket dimension wasn't fused with another parent universe in it's creation,this is possibly due to the fact that it could be a dream dimension .
Wisty Willow could have originated from an abstract dream. The island seems to have no inhabitants,the water is bottomless and has no sea life. More evidence of Wisty Willow being a dream world if you go deep enough into the water,the area around you will begin to distort and defy physics. Such as "pancake fish",time distortions,etc.