Helen: The world wasn't created in a deluge, it was covered by it.
Some guy: That's blasphemy!
These lines always nagged at me. I think it means the people hold a religious belief that the Earth is ONLY water, a liquid planet, like a raindrop. So to answer your question - dogma.
As an aside, this movie is the first place I saw a woman's bare ass. Ka-ching!

In one scene the mariner says that the map is of the cities down below and that the poles have reversed. When he said that dry land was below, someone yelled, "That's blasphemy!" Perhaps indicating that even considering there might be land below them had somehow become forbidden.
Also in the face of such a great catastrophe the human race would probably be in survival mode for generations, focused mainly on staying alive and their children staying alive.

There are cultures that have oral traditions that have recitations and chains of narrations that go back thousands of years ( middle eastern, native ect.) and there are cultures that dont have oral traditions, i assume word got around from travellers with said oral traditions.

That's not true. Apparently Harrier Jump Jets can be operated 1000 years after the last maintenance was done one them:
http://www.imdb.com/board/10185183/?ref_=nv_sr_1
SpiltPersonality

And it cannot be that long, planes and boats would not be functioning after less than 50 years.

Even though they showed it a lot in the movie it was said that paper was very rare. So it's possible most people had never even seen it.

I assumed they knew, there were old magazines with pictures of land.

The film's setting is largely a fantasy, designed to atavistically depict our present as a myth and the film's present as a new Dark Ages. As the miserable inhabitants of the Atolls are presented, humanity may well be on the path to extinction, barring some last-minute 'Deus Ex Aqua' revelation of actual dry land that will 'save' the human race and set it back towards a terrestrial culture, which is, in fact, what concerns much of the film's storyline.

I think so seeing as how it was likely they had no point to give hope to what was literally nowhere to be found without the tattoo's indications.