You found pretty much everything. The border you outlined is nearly perfect and I'm in the process of blocking off the region completely. It will be visually blocked by impassable mountains with invisible walls also in case you find your way through. Blocking off the water immersively will be a bit more of a challenge. Possibly might add extreme cold so you'll freeze to death if you go to far. (You'll have time to go back to shore once you realize you are freezing to death) As new regions open up, passes in the mountains and rock formations will open up to make the new areas accessible. Once all regions are open, the dragon will grow to "rideable" size.  I may figure out a way to do that sooner before all regions open. It's a tough one. I don't want to just have invisible walls in the sky to fly into, but that may be the case.

One thing you may have missed. There are 6 cabins total. 5 locations (one has two cabins)

There are also 3 caves with loot (not including the one with the dragon egg) and I'm going to add a handful more to the starter region in the next update. 

Thank you again for all the playtesting and feedback. I've comprised a list of many things to work on and hope to have an update soon.