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If we lived on a planet similar to Earth with the same lengths for day and night - but the entire planet was covered (at some point in the year) by water, could civilisations exist and prosper as they have on Earth?

My visualisation of this planet is something similar to Earth except it is entirely covered in water at some point in the year, but a force other than the sun continuously pulls the tides around the planet. The tide takes an entire year to come around fully, so there would be a few months on any point where there would be no ocean, then it would be submerged for a few months when the tide comes back around. There is a lot of water on this planet but a large gravitational force is able to keep nearly all of it moving, leaving only some lakes in the imperfections of the land. While there is a lot of water, the slow-moving nature of it doesn't erode away the land extremely quickly as it just slowly moves around the entire planet. Perhaps a moon that somehow moves very slowly can create this force. When the water is on part of the planet it can range from being a few metres deep to up to half a kilometre, and when the "tide" has been gone for a few weeks the surface could appear very dry again depending on the distance to the equator.

Assuming human life has managed to evolve on this planet, would it be possible for them to build a long-lasting civilisation in this world - perhaps a maritime nation with no stationary home but a constantly moving fleet, or would the constant moving and difficulty to find resources and forge technology simply make this too hard to form civilisations?

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    $\begingroup$ So the water of the whole planet constantly moves from one side of the planet to the other? In a single day? does that mean that the surface is just extremely eroded and more or less flat? $\endgroup$
    – Nuloen The Seeker
    Dec 8, 2017 at 20:33
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    $\begingroup$ I think the last sentence might invite people to vote to put this question on hold until it's edited, because it's inviting brainstorming, which we often call "primarily opinion-based" here. Basically all answers on "What do you think?" are equally valid and there needs to be a way to somewhat reliably vote which answer is the best answer out of all those that are given. I'd recommend removing that part and focusing on other stuff that can be answered somewhat objectively. Just as info: when 5 community members think a question doesn't fit it can temporarily be put on hold until edited. $\endgroup$
    – Secespitus
    Dec 8, 2017 at 20:36
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    $\begingroup$ Are you asking could humans live on such a planet or if humans could evolve on such a planet? There are a lot of ambiguities in this question that need to be clarified before it is really answerable. I'm voting that this question gets put on hold until you edit the question to clarify things. $\endgroup$
    – sphennings
    Dec 8, 2017 at 20:45
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    $\begingroup$ I like this concept but think the question should be changed from what would human civilization look like., to something more specifics $\endgroup$
    – Garret Gang
    Dec 8, 2017 at 21:18
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    $\begingroup$ You ask many different questions in this post "could a civilisation exist?", "could life exist?", "Would the life be similar to earth's?", and "What would the civilization look like?" Please limit yourself to one question per post. $\endgroup$
    – sphennings
    Dec 8, 2017 at 21:38

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A partial answer is how to make this happen: assume the moon is in a near geosynchronous orbit around the planet. That is, the moon's orbital period is almost identical to the planet's rotation period. For Earth, this means the moon should have been closer. The tide will be stronger and circumnavigate the planet very slowly. Just don't forget that tidal forces don't act only on the water. They flex the planet's crust in the same direction. This makes the crust slightly rise and fall with the tides, mitigating water-level changes to an observer on the surface. Slow-cycled strong tides affect the crust more than fast-cycled weak ones. Expect many earthquakes too.

All land animals should have evolved means to cope with this, so by the time intelligent aliens evolve, they must have already adapted. Adaptation may have come in many forms, including ability to swim over long distances or retaining amphibian way of life.

Higher latitudes experience weaker tides, maybe allowing few landmasses to remain permanently above water. Did your humans find refuge in high latitudes and high elevations?

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  • $\begingroup$ It's also important to note that tidal forces creates a second bulge opposite the moon as well. $\endgroup$
    – Stephan
    Dec 9, 2017 at 17:43
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The impression I get from your description is that most points on the planet are submerged half of the year, then in dry land the other half of the year (until the huge slowly rotating mass of water comes back around).

It is totally possible to grow a variety of crops in less than half a year. Assuming during the "dry land season" you still get access to irrigation water and fertile soil, with good insolation and diligent farmers to take care of the crops, there is plenty of opportunities to produce various food stocks for the "floating season". Note that in this scenario sediments following the passage of the "huge massive wave of water" (dead whales proteins) might make lands more fertile.

Another - less realistic - possibility is living on "floating islands" somewhat engineered to be able to sustain habitation and cultivation (on platforms covered with soil). These islands would follow the huge wave.

You mentioned a planet "similar to Earth" so I assume no contact with a neighbor planet, otherwise you can also consider a society relying on importation of food from another close planet.

For inspiration, I can suggest reading chapters of the first half of the (non-fiction) book "Collapse" by Jared Diamond, that describe how very isolated societies have been able to live for several centuries, under very harsh or restricted conditions ; and which are the conditions that were critical in their survival (or collapse...).

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