There's a fight in this poem between light and dark, harmony and chaos (kind of makes it sound like Star Wars doesn't it?). That fight doesn't just happen on the level of ideas and grand concepts though. We hear it in the sound of the words, too.
In the first couple of stanzas, we hear the smooth, calm rhythm of the waves, washing in and out, soothing us with soft sounds. Take line two for example: "The tide is full, the moon lies fair." Two matching, balanced phrases, in perfect iambic meter: daDUM daDUM, daDUM daDUM. The sound of the words "full" and "fair" is as relaxed as the meter and the imagery.
Shaking Things Up
Then, slowly, new sounds start to creep in. The word "grating" in line 9 is maybe the first sign of trouble. There's nothing happy or calming about a grating sound, and it breaks into the easy tidal rhythm of the first few lines. It's one of those words that sounds a little like the sound it's describing (the fancy word for that is onomatopoeia). This trend picks up speed in the rest of the poem, as we get more lines with broken, strange meter and harsh-sounding words, like "naked shingles" (28).
Finally, in the last lines, as the chaos of the world takes over, that chaos seeps into the sound of the poem as well. The rhythm of the waves has been taken over by the harsh clanging sounds and disrupted rhythm of battle and fear. Take the last line, for example: "Where ignorant armies clash by night" (37). The words and the sounds are harsh, and the iambic meter is gone. Words like "ignorant" and "clash" attack our ears, and the poem's transition to chaos is complete.