It may seem from the Pesukim that the actual height of the water above the ground was 15 Amos while the powerful waves (as would be expected in such a high flood) raised the water up to the high mountains. This would explain a lot of questions, including how part of the world can be unaffected while the mountains are submerged in another part of the world.
If animals came from a certain part of the world it only makes sense that they went right back. Noach was a man of the earth (who observed and took note of animal traits and behaviors) and would not be so sloppy as to pour everyone out in one area. We would not do that today.
Since Chazal say that he took along Sheidim, this sounds like it did in fact affect the whole world, although the fact that they mention Shekker and Chazav makes it sound as if something else was happening. If some continents were untouched Og could have gone there. I don’t think that the fish surviving was such a Nes since they could have been far away from the top waters.
(The unspoken message in the mind of many is that Torah Min Shamayim is an allegory. Isn’t that the most bizarre of all the Nissim?)