The Ending of "Foe"

I was somewhat confused by the ending of Foe. I definitely did not see it coming, but I can see how the ending makes sense, considering what we have discussed in class. We've talked about how Susan Barton is a construct of Foe and Foe is a construct of Susan Barton, but in the end it is revealed (I think) that Foe made up the whole story with Susan Barton. (I think) The truth is that Susan Barton was dead all along/never existed. However, I thought it was interesting how Friday's "voice" turned out to be all the sounds of the island, which is a very powerful voice. Friday's "home" is underwater in the sunken ship because you can't speak there. But what does the transition from walking in the house and finding Friday's body, to being underwater and finding the sunken ship mean? And why is Friday in the house at first, but then in the sunken ship? Maybe I'm not thinking about it on a deep enough level, but I don't really see the meaning behind the ending. I could see how it could be representative of the revelation that Susan Barton and Friday didn't actually meet Foe, and maybe even that Susan Barton and Friday never even existed.

Even though I can see how this makes sense, I still just don't really get the ending. I can't really explain, but I don't understand Coetzee's purpose of structuring the book this way. Maybe there isn't a purpose, maybe the purpose was simply the fact that nothing that "Susan Barton" wrote actually happened.