Break, break, break, On thy cold gray stones, O Sea! (1-2)
The repeated "break[ing]" of the waves of the sea suggests the inevitable progress of time – you cannot stop time from moving forward any more than you can stop the tide. The repetition also suggests that time passes without changing much.
The fisherman's boy (5)
The presence of a child in this poem about grief and death seems important; it could suggest a kind of hope for the future, since that happy "fisherman's boy" and his "sister" are going to grow up.
Break, break, break At the foot of thy crags, O Sea! (13-14)
These lines repeat, almost exactly, the first two lines of the poem. The repetition suggests that time passes without changing anything, but the difference in phrasing of line 14 ("at the foot of thy crags" instead of "On thy cold gray stones") might be more hopeful. The difference might suggest that as much as time seems to repeat itself with no change, year in and year out, there are subtle changes happening all around you.
But the tender grace of a day that is dead Will never come back to me (15-16)
Because time keeps moving forward, the speaker realizes that there's no way he can go back to the time before his friend died – those days are now "dead." It's rather a grim way to describe the passage of time.