There should be a good mix of rock materials, some of high density and some of low density. Then the low-density parts of the crust will be thicker than the high-density parts. Since the crust will float on the molten core, the thick parts of the crust will be continents. Their outer surface elevation will be higher than that of the thin parts of the crust, which will become the ocean basins. Such a mix will provide a variety of habitats. Otherwise, a planet with much water would be almost all ocean, and one with little water would be almost all continent.