"The Seafarer", like most Anglo-Saxon or Old English poetry (including Beowulf) is written in what is known as strong-stress or alliterative meter.
Unlike modern English poetry, in which feet are composed of a fixed number of unstressed and stressed syllables, Old English poetry is purely accentual. To analyze the meter of accentual poetry, one counts only the strong syllables and not the weak syllables.
In "The Seafarer", there are four stressed or accented syllables per line. There is a caesura or pause in the middle of each line, dividing the line into two hemistichs, with two stresses falling on each side of the caesura.
The first stressed syllable of the second hemistich must contain the same initial consonant sound (i.e. alliterate with) one or both of the stressed syllables in the first hemistich.