@Vaqvbu1a
Really -_-
Your three(only two) sources are desalination, and the underground water near the coast. I disapprove of desalination because of the effort:reward ratio being too small. So that leaves the underground water. I talked about saltwater because the air near the coastline carries it. Even if he uses freshwater to water the plants with it being at the coastline the air will still bring a certain amount of salt to the plants.
The efficiency:reward ratio I am talking about is the
MC desalinating water. NEVER have I said humans. In HIS circumstances and the amount of what is shown in the manga he can't make a stable desalination of saltwater to support himself while developing the land. plus that and his lack of tools and/or knowledge to build what he needs.
I'll leave your first point alone since it is your statement.
well he did it in the manga so it is possible in that world. Also we don't know the sizes of the planets to assume gravity in that world. The only shown thing is that he dug deep and nothing fell. If it did work like in the real world then it would have collapse when he dug least around twice his height so not much danger for him there.(Since he can just use that idea he has with the shovel shield).
You can't prove the manga world with graphs from real life. In that sandy beach he would need to find the nearest solid land first and we don't know the topography of the place. We can't prove it in the drawing since it is not detailed enough. Ok lets take a step back and assume he doesn't know any of the stuff we are talking about. It would make sense that he went to the wetlands. Is he retarded for not knowing that? NO, because a city otaku with all that convenience doesn't need that knowledge.
Wasteland is not considered inhabitable. Inhabitable is a place where you can't live in like the moon or underwater. Wasteland is more like nothing lives there anymore or if there is something that lives there it is the extreme minority like cactus in the desert. Water is can be found even if it is a wasteland.
The 100m is just my curiosity since regardless we won't know which place has a closer freshwater unless he digs at the coastline for comparison.