As for the reflection of the cityscape in the water........
The producers have made several references to mirror images. In last nights repeat of episodes 22/23 "Through the Looking Glass" the reference was made in the notes running at the bottom several times....once during Jacks flash forward at the beginning of the show. They spoke of him being a "mirror image" the exact opposite of what he was on the island.
On the island Jack was a leader, he was strong, and determined. In the flash, he was weak, indecisive, ready to give up, and alone.
So, to me the city in the water represents that mirror image of what they will be if/when they return to that world.