It looks so simple, this poem, that you might skip right past how it sounds to get at what it means. But if you do that, you'll miss a big ol' treat because "Not Waving but Drowning" is chock-full of sound effects for your enjoyment. It's best to read the poem out loud, slowly. Seriously. Relish it, bellow it, make your parents or your roommates worry about you a little bit.
Alliteration
One of the first sonic tricks we hear in the poem is line 1's repetition of "h" sounds at the beginning of "heard" and "him," but that's nothing compared to the "h" parade huffing through line 7: "have" and "him his heart." Most of these repetitions of initial letters happen within the lines, giving the sound of each line more unity (and making it more fun to say). Smith uses alliteration sparingly in this poem, but see if you can find another good example.
Consonance
It's not just initial letters that repeat. Take a look at the consonants that keep showing up through the poem. There's the thudding "d" sound in "nobody," "heard," and "dead," which could mimic the crashing of waves, or even a heartbeat. Listen too for the soft "th" sounds in the first stanza—"further," "than," and "thought"—and hear how they clash with the harder "t" sounds throughout the poem. Could these be the waves whispering at first, then getting choppier? You could say the poem is swimming in repeated consonants.
Assonance
The repetition fest ain't over yet. Repeated vowel sounds pop up throughout the poem to link one line to another. For example, the first word of line 1, "Nobody," links to the last word of line 2, "moaning" through the same long "o" sound. In fact, you'll find this long "o" throughout the poem. We think it represents the sound of the dead man's moaning perfectly. Look, too, at the "ow" sound in "out" and "drowning". They're tied together by what sounds like a cry of pain, right at the moment the dead man describes his suffering.
Internal Rhyme
Finally, in a few places the sound repetitions come together to give the poem more exact rhymes. Since the ballad stanza only gives two end rhymes per stanza (see "Form and Meter") and this poem makes two of the three rhyming pairs feminine rhymes, we have to look inside the lines to find the cleverly hidden precise echoes.
See, for example, the way the end of line 3 ("thought") rhymes with "not," the second word of the next line. Or check out line 7's "way" chiming with line 8's "They." As with the assonance, internal rhyme works to connect lines to each other in a more subtle way than the usual end rhymes would, making the poem like an awesome echo chamber.