This relates to the bible concept of firmament, flat earth and separation of waters, as in genesis when it says god separated waters above and below.
The nomads knew wells and rain and islands, so the world they conceptualised was one where God separated those things to reveal dry land. As such in the story massive amounts of water could well come from below returning the Earth in what they considered a previous, erased state to reboot it.
This relates to the bible concept of firmament, flat earth and separation of waters, as in genesis when it says god separated waters above and below.
The nomads knew wells and rain and islands, so the world they conceptualised was one where God separated those things to reveal dry land. As such in the story massive amounts of water could well come from below returning the Earth in what they considered a previous, erased state to reboot it.