The Sea
There are a lot of things you could say about Longfellow's poetry, but one thing for certain is that, in his later years, he loved to talk about the ocean. By ocean, we don't just mean "ocean," but the waves, the tides, the shore—all that other business typically associated with the ocean. "The Tide Rises, the Tide Falls" is a great example because you've got the tides, the waves, the sea, and the shore, all in one poem.
In the dedicatory poem that Longfellow included at the beginning of Ultima Thule, the speaker talks all about how the "ocean streams" (5) have carried him so far from the pleasures and carefree days of his youth, while in "Becalmed" the speaker waxes metaphorical and talks all about the "sea of thought" (1) and the "life of the awakening sea" (15).