The names give away their locations. The oceanic layer, underneath all of the oceans, is only 2.5 to 3 miles (4 to 5 kilometers) thick at its deepest and is very dense.
The continental crust can be up to 40 miles (64 kilometers) thick. It is not as dense as the oceanic crust, so it is more buoyant, or capable of floating. The continental crust extends out into the oceans up to 200 miles (320 kilometers) before sloping off.
This continental crust under water is known as the continental shelf.