I've looked around the forums, and I'm not the first to be sharing this; though, I thought I would give my input. I found this uncharted island right in the middle of the map. There are shipwrecks and a giant (kraken?) skeleton, as well as some "man-made" structures. I put my screenshots all in this imgur...check it out:
I'm thinking there will have to be some significance in the full release.
EDIT: 3/2/2018 - I can confirm more of these islands exist in between the biomes. They seem to all have some reef structure littered with shipwrecks. In addition, they all have these man made structures. It seems only the central one has a kraken skeleton. Well, I found one and swam around.
...and I swam under the island...and found this!!! :
Similarly, the one at N10 has this underground area:
So all together, I found 3 thus far. N10, L15, and S16. In the middle of this map is some very large rock structure pictured in the first imgur album.