The central idea of the poem concerns the dehumanizing power of uncontrollable grief.
The speaker stands before the violent waves, enjoining them to continue crashing hard against the rocks. He may not be able to express himself, wracked as he is by the most unbearable sorrow, but the waves can still do what they've always done, rising up to smash against the shore before retreating back towards the horizon.
Initially, the speaker gives the impression of being rather envious of the waves' extraordinary power; if only he could be as strong as they. Yet in due course he seems to realize that the endless movement of the sea and the ships that sail upon it is largely pointless.
Whatever happens in his life, whatever suffering he may endure, the sea and the commerce that it enables will still carry on regardless. In that sense, the speaker is in the same boat as the sea, simply going through the motions of his daily life without thinking about them.
Grief has dehumanized him to such an extent that he's become little more than a feature of the natural world, just like the sea, but without its extraordinary power and without its mute indifference to grief and suffering.