A good portion of the poem takes place on, near, or under the sea. The sea is a lot of things here: a threat, a means of transport, but, most importantly, it's a source of beauty. Go back and check out all the wonderful colors and descriptions that Pound uses to paint the sea for us. In a poem that asks us to re-examine our fundamental attitudes toward beauty, Pound goes all Bob Ross and uses sea imagery to paint an ideal of beauty in the natural world.
Lines 5-6: As with "Canto I," Pound starts off "Canto II" by putting the sound of the sea in our heads. He even makes up a new sea god with a name that sounds like washing water, claiming that "So-shu churned in the sea" and giving us a good sense of the wishing and washing of seawater.
Line 13: When he mentions the blind poet Homer, Pound makes sure to include the thought that Homer had an "ear for the sea-surge" (13). In other words, Pound is saying that Homer's blindness might have given him an uncanny power to write poetry that sounded like the sea, since Homer lived near the sea and would have paid more attention to its sound because of his blindness.
Lines 121-122: After telling the story of Dionysius and Acoetes, Pound switches to a myth he made up about a maiden named Ileuthyeria, who escaped a bunch of mermen in the sea by being transformed into ocean coral. That's why Pound claims that if you "lean over the rock" (121) and look into the ocean, you'll see her "coral face under wave-tinge" (122).
Lines 130-131: Ever the musical poet, Pound returns to the phrase saying "So-shu churned in the sea" to bring all of our attention back to the sound of the ocean, which Pound never wants us to stop hearing throughout this poem. For Pound, the ocean seems to be a place of endless beauty and creation, even though it's a giant void. So having the god So-shu stirring up the sea and "Using the long moon for a churn-stick" (131) makes us think of the sea as a giant pot, out of which comes all the fantastic creations of poetry.