"Break, Break, Break" has lines with varying numbers of syllables. The metrical feet also vary. For instance, the first line has only three stressed (long) syllables and this is called a molossus. There are two anapests in the third line but they are followed by an amphibrach. The two anapests, (two unstressed syllables followed by one stressed), are "And I would" and "that my tongue" but "could utter" is an amphibrach (unstressed, stressed, unstressed).
The second and fourth lines of the second stanza are strictly anapestic, having three anapests:
That he shouts with his sister at play!
That he sings in his boat on the bay!
The last line of the third stanza is also strictly anapestic:
And the sound of a voice that is still!
The third line of the final stanza is almost strictly anapestic. It is an anapest followed by an iamb and then followed by two anapests.
Notably, the three strictly anapestic lines describe people's voices: a fisherman's son, a sailor, and the "sound of a voice that is still" which refers to an absent ("still") voice, the death of a friend. Most critics note that Tennyson wrote this, and other poems, about his friend Arthur Hallam. The waves continue to break but the speaker (Tennyson, considering the poem as biographical) cannot relive the past, the time when his friend was alive.