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Worldbuilding

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    $\begingroup$ @Michael 20m is nothing even compared to what the Earth has experienced in the past, when the moon was closer. Hundreds of metres ought to be plausible. I'm talking about something resembling a major tsunami every day, which would just wash away any wooden jetty. The main limit on tide height is that if the tidal forces are too strong the crust can melt and you get a magma ocean instead of a land surface. (That has also happened on Earth in the past.) If this was hard-science I'd see if there was a way to put numbers on that, but for science-based I think it's ok to handwave it. $\endgroup$
    – N. Virgo
    Commented Dec 1, 2020 at 10:52
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    $\begingroup$ Note for all - I was very shocked to find that our once close moon, with enormous tides, quickly moved away to much lower tides in one hundred years. That is a blink on a geological timescale. Very surprising (and dashed many of my theories on early evolution!). I had imagined millions of years of tsunamis but that is not the case. I do not know if the slowdown is mostly due to the change in spin or the effect of massive oceans slowing it through tides $\endgroup$
    – Michael Durrant
    Commented Dec 1, 2020 at 10:59
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    $\begingroup$ @MichaelDurrant thanks, that's useful information, and it makes sense - the energy for those huge tides has to come from somewhere, and that somewhere is the gravitational potential energy between the Moon and the Earth - which gets dissipated by the moon moving further away. So to make this realistic the planet probably would have to be a moon of a gas giant. (Io experiences much higher tidal forces than Earth - though it also has too much volcanism to be habitable, so the forces would need to be less than that.) $\endgroup$
    – N. Virgo
    Commented Dec 1, 2020 at 11:08
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    $\begingroup$ Overall I really like this answer, but there's something that undermines its validity: rivers. Rivers necessarily end up in the oceans, and the situation described in the answer doesn't prevent building littoral infrastructure on lakes and rivers. From there on out, there's a technological evolutionary path towards building ships that can survive in tidal basis, and eventually ships that can sail into the ocean. You could perhaps improve the answer with ways that prevent navigable rivers. $\endgroup$
    – Thierry
    Commented Dec 1, 2020 at 14:56
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    $\begingroup$ The height of tides is extremely influenced by geography. Ex Earth has a 0.6m tidal difference on the open ocean but 12-15m in the Bay of Fundy and almost 0 in the Mediterranean $\endgroup$
    – user3067860
    Commented Dec 2, 2020 at 8:32