There's a mood that runs through the whole poem, and a particular sound that we hear over and over. It's a kind of gentle, rhythmic, drowsy sound, like a calm wind breathing through the poem. Actually, maybe Tennyson says it best: "All round the coast the languid air did swoon, / Breathing like one that hath a weary dream." (5-6). He really follows through on this idea on a sonic level, too, keeping up a gentle, quiet sound that almost puts us in a trance. Listen to a line like this: "Rolling a slumbrous sheet of foam below" (13). Can you hear the repeated long O sound in "Rolling," "foam," and "below"? That's assonance for you. And how about the soft, slithery, sleepy S sounds of the "slumberous sheet"? Say hello to our little friend alliteration. So what's the effect of these sound techniques? Well, we get a line that's super-relaxing and kind of hypnotic.
This is just one line of many in which Tennyson uses sound to mimic or highlight the content of the poem itself. Even if it's not all beach hammocks and chillaxing under a palm tree, the poem's use of sound follows the action of the content. Just check out the consonance of the repeated R, L, and D sounds here: "Roll'd to starboard, roll'd to larboard, when the surge was seething free" (151). The sounds here bump into each other like so many sailors tossed about in a storm. Just as he paints a picture through narrative and description, Tennyson is making those images pop in our minds with a ton of clever sound accentuation here. The dude is like a verbal Bob Ross.