The destructive force of waves is not felt while they are out at sea, in the open ocean, but it is

felt when they do what?

The destructive force of the wave is not felt while they are out at the sea but it is felt when they approach the shore.

Explanation:

When the waves are out in the ocean they are in deeper waters. Once they move towards the shore they approach shallow waters. This movement causes shoaling of the waves by which its amplitude increases.

When the wave reaches the shore its leading edge gets slowed down by coming in contact with the beach floor but its trailing edge which is still in deep water still moves rapidly. The wave thus gets compressed and its wavelength decreases. The amplitude of the wave then increases rapidly making the wave destructive.