The images of the sea in stanza 1 and stanza 4 are quite similar. In the first stanza, the "break" is repeated; as it is in stanza 4. This stresses the ongoing breaking, that the sea is (cea)seless. In stanza 1, the waves crash onto stones and in stanza 4, they crash into rock cliffs. But even these harsh objects can not stop the sea's motion. Contemplating the sea's never-ending cycle, the speaker is grieving all the more because what he really wants is for time to go backwards: so that he might see the person he is grieving for. We know from stanza 3, that someone he cared about has died: "vanish'd hand." In both stanza 1 and stanza 4, the sea reminds the speaker of the ceaseless movement of time and it vexes him even more because time separates him more and more from the one he has lost.
In the last two lines, the "tender grace of a day that is dead" refers to the last time his loved one was alive. That day is gone. The last line underscores his grief: that he will never get that day back, because time moves on. The feelings are grief, but also frustration and even anger at time itself.