As you may have guessed by the title, the poem relies pretty heavily on some sea-worthy imagery and symbolism. In part one, for example, the speaker wanders the shores of his hometown, musing on how the sea is more than just a body of water. In this poem, that water works as a metaphor for life.
Line 3: "The ripples [are] continually wash[-ing]" the shore; in fact, the entire poem is filled with imagery that reinforces the idea that the ocean is a repetitive presence in the speaker's life.
Line 21: To the speaker, it's "the ocean so mysterious," indicating that part two of the poem is about the oceans that don't feel like home. Is there a difference? Not really, says the speaker; he spends much of the rest of the poem discussing the ocean as a symbol for all life. They combine to make one entity.
Line 34: Here we get this unity: "You oceans both, I close with you." In fact, this even symbolizes the speaker himself, who sees himself as part of the ocean—and all of life's creation.
Line 41: "I too have bubbled up, floated the measureless float, and been wash'd on your shores" the speaker repeats. He's part of the ocean for sure. And, by the end of the poem, the sea metaphor has stretched to all of creation.