"Shipwreck" uses assonance to make its images more vivid.
In line 2, for example, the speaker describes the boat as a "little brig." That quick, light /i/ vowel helps to emphasize the boat's smallness set against the vast backdrop of the ocean and the overwhelming power of the storm. If the line instead read "little boat," say, the effect just wouldn't be the same!
The same sound reappears in line 6:
It slipped and slipped,
Here, the /i/ quickens the line, mimicking the acceleration that comes with slipping and falling.
The poem really dials up the effect in lines 11-12:
Ah, brig, good-night
To crew and you;
The ocean's heart too smooth, too blue,
All those /oo/ sounds in a row smooth the line out, emphasizing the image of a calm ocean's vast, featureless surface. This supports the image of the ocean's smooth, blue heart, which is too indifferent to "break"—that is, to feel—for the brig and its crew.