Render the scene. There is no displacement! That is because we don't have enough subdivisions in the geometry of the sea plane.
Select the plane and under the Arnold attributes, change the subdivision_type to catclark and increase the number of iterations. Start off with a low number like 3:
Subdivision Type set to Catclark, Subdivision Iterations subdivision_type: catclark, subdivision_iterations: 3
The sea looks ok, but it really needs more detail. Increase the subdivision_iterations to 8. Be careful not to increase this value too much as memory use and render times can increase dramatically with each increment (each iteration quadruples the geometry).
Subdivision Iterations subdivision_iterations: 8. More detail can be seen in the sea waves.