Inspired by the 2014 film Interstellar directed by Christopher Nolan, the crews landed on a planet with water flooded up to knee level. Suddenly a wall of water like mega tsunami appears from a distant and approaches them very quickly, actually this phenomenon was explained by Kip Thorne, the science adviser for the movie that since the planet is orbiting a black hole, a type of stellar remnant whose escape velocity exceeds speed of light in other word it exerts strong gravitational pull to cause the water on the planet to bulge sideways. As the planet rotates on its axis it would appear like a mega tsunami sweeping the surface, let's ignore the exaggerated effect of the ocean tide as depicted in the film and grant it a one time scientific miracle and say there are signs of intelligent life on the planet that somehow orbit around a black hole unscathed. How would any land dwelling mammalian intelligent life had evolved to accustom/adapt to such an hostile environment/world? Note: the tide can carry enormous amount of energy to sweep even concrete structure remember a black hole is no moon. I like to borrow the same plot and throw a welcome party for my story.
Given the force of the waves going around, I think any land not on the poles will have been eroded long ago, which probably explains the fairly uniform submerged land we saw in Interstellar.
Now a lot would depend on the tilt of the planet's axis relative to its orbit. If the axis is perpendicular, the poles would be fairly safe and stable, as the water is being pulled away from the poles toward the equator. Mammals could have evolved there, either living in caves or very resistant to all the radiation the black hole's accretion disc would produce. As Jane S mentioned, the edge of the land would be a big focus for them, scavenging for food and resources along the tide pools left behind by the daily waves.
Things get more interesting if the axis is tilted differently. This would mean that there are seasons during which the poles are submerged to a different degree. Our Summer/winter might be their flooding season and spring/autumn their dry season. I imagine this would lead to seasonal spreading out onto the drying land, grazing herds/gathering food, then retreating to the highest mountains to sit out the flooding again. These seasons might only last a short time if the planet's orbital period is very short.
Ultimately, all the land would disappear, as the waves erode the edge of the polar land and the currents wash the sand towards the equator, raising the sea level, etc. If the species already achieved some level of technology, they probably have built undersea habitats or sturdy floating rafts.
Some more ideas:
- The locals might migrate from pole to pole on rafts/boats if they are submerged alternately, following the seasons. I don't know whether the seasons could be long enough for that though. Maybe the axis wobbles due to gravitational effects. This might even be irregular for extra flavor.
- The last land is going to fall into the waves within a few years and the locals have been trying to develop some survival method, but until a spaceship landed, they had never thought of escaping from the planet.
- The locals have near unlimited electricity from hydroelectric plants. They use this to grow their food, heat their houses, perhaps even use force shielding to protect their city from the waves.
With the violence of the waves across the surface, you probably wish to avoid these areas for living on the surface of the water. So as far as I see it, you have two possible civilisations (perhaps both exist):
You don't state the depth of your water, but your comment implies that there may be regions of deeper water. If the planet is tidally locked, then perhaps there are vast regions that are deep enough for a civilisation to develop.
Again, you don't exactly state if the land area remains unaffected by the tide, or at least a portion of it. If there is enough area here for some sort of flora and fauna to exist, then there should be sufficient space (perhaps) for a more advanced society.
How to use the tides as part of society?
There are a couple of options:
- The land society could may have resources from the ocean floor dumped onto any "beaches" that may exist. Given the force of these waves, there will almost certainly be a flat transition from the water to the land mass.
- Entertainment/Coming of age ceremonies. Perhaps either society could use the huge tides and surfing on them as a trial to enter adulthood. Or simply use them as entertainment. Or they could even float up the front and launch gliders from the top. There are lots of possibilities.
- Timekeeping. Perhaps the sweeps of the tides could be used as a way of tracking time, depending on their frequency (if Interstellar is any sort of indication of the that in your world).