One major difference between oceans and land occurs in their respective crust formations.
Oceanic crust is in a constant state of renewal. New crust material forms out of ocean vents where the crustal plates are in motion; where the plates push against each other, the crust is pushed up, while where the plates pull away from each other, magma from beneath the crust bubbles up and becomes new crustal material. In addition, the ocean itself constantly moves the lighter and new material around the ocean floor. Because of this, the ocean crusts are simple in formation, containing fewer minerals and elements, while also being both thinner and more dense than continental crust.
Continental crust changes much more slowly. Since there are fewer strong forces affecting the continental crust, the land is free to develop sedimentary layers unchanged by the constant movement of the oceanic crust. Continental crust is both thicker and less dense than oceanic crust, and contains many more minerals and elements because of the long development times without constant movement or renewal. This means that the continental crust is a safer place for permanent structures, as they are less likely to be destroyed by the constant movement than on the ocean floor.