Alliteration is used throughout "Diving into the Wreck." This can draw connections between words, focus readers' attention on certain phrases, and evoke a line's content through sound.
The first clear example is in line 5 in the /b/ sound of "body-armour" and "black." These are strong, plosive sounds that momentarily stop the airflow if read aloud. These gives them a kind of slapping quality, like a wetsuit being pulled on tight and pinging into place. The sounds draw attention to the words, which make it seem like preparing to dive is akin to preparing for battle.
Later in the first stanza, the /s/ alliteration (a.k.a. sibilance) of "sun-flooded schooner" gently evokes the sounds of the ocean's surface. Think about the noise of waves, and the soft splash of water breaking against a boat. This is picked up in the second stanza as well, while the speaker is still preparing for the dive and mentions "some sundry equipment" in line 21.
When the speaker begins to make the descent down the ladder into the ocean, line 23 use alliteration in the phrase "rung after rung" (also an example of diacope). This gives the impression of a step-by-step action, evoking the careful but purposeful way that the speaker begins the dive. The harsh alliteration of "cripple" and "crawl" in lines 29 and 30 then draw attention to the difficulty the speaker faces while moving down the ladder in flippers.
In the following stanza, a quick burst of alliteration gives the poem an anxiously excited sound as the speaker heads into the water:
First the air is blue and then
it is bluer and then green and then
black I am blacking out and yet
my mask is powerful
it pumps my blood with power
the sea is another story
The alliteration above suddenly quickens the pace of the poem, giving it a breathlessness that fits with the somewhat frightening transition from surface world to ocean depths.
Another striking example is in line 55, when the speaker clearly states the purpose of the dive:
I came to see the damage that was done
These two /d/ sounds have a heavy effect, subtly evoking violence and destruction. Then, in the following stanza, sibilance again evokes the watery underworld explored by the speaker in "staring," "sun," "salt and sway."
Looking at the ship, towards the end of the poem, the speaker identifies with its abandoned treasures:
whose breast still bear the stress
whose silver, copper, vermeil cargo lies
The alliteration here perhaps pushes readers to linger on these lines, to better appreciate the abandoned bounty before the speaker. Finally, the final stanza features another striking moment of alliteration with the hard initial sounds of "courage" and "cowardice." Alliteration binds these opposite words together, revealing that both bravery and fear can push someone down towards the wreck.