A large group of people has been banished from Earth and punished to scrape out a perilous existence on a strange planet. The planet has breathable atmosphere, and consists of an immense ocean teeming with wondrous marine species (some of which are catchable and edible), and small islands far apart from each other that are mostly barren and with very little flora and fauna. Partly because they need a sufficient source of nourishment, and partly because of their large numbers and need for living space, people are compelled to live near the shore. They have a problem, however:
The planet has incredibly high tides. Accordingly, the shoreline moves many kilometres back and forth with the tide while the sea level rises and falls hundreds of metres. At low tide, there is a considerably larger area to live in but food is far away. As their dwellings need to be on dry land and they have no resources or tools with which to build floating vessels, at high tide, the living and breathing space becomes so cramped that people start suffering from phobias and can barely stand each other’s presence.
In order to survive and not lose their sanity, people will have to adapt. There is a plethora of unexplored questions concerning the society that will emerge as well as the abilities these humans will develop. The first question, however, has to be:
What kind of planet could have such extremely high tides? What unique features do its geosphere and hydrosphere and/or planetary system need to have?
Thank you for your help!
Note: If you find the living area constraint unrealistic, please consider for instance an island that consists of a high plateau rendered unreachable by fabulously tall vertical cliffs. (I am not sure how geologically possible this is, but I guess this can be dealt with in another question :). The humans live at the foot of these cliffs and during high tide there remains only a narrow strip of dry land.
In any case, while researching, I found a nice simulator of what it would look like to have a higher sea level on Earth.