The SeQish homeworld has the same name as it's primary inhabitants. Ninety percent of the planets surface is covered with water. There is only one large continent, and several hundred medium to large sized islands, along with several thousand small islands.
Though SeQish only has one major continent, there are several different tectonic plates at different heights in relation to sea level. There are many "undersea continents" that sit, on average, approximately 100 to 200 meters below sea level.
These "undersea continents" are where the SeQish people build their underwater cities.