This is a really interesting question. In actuality, the speaker doesn't see anything. He is recalling all of the aspects of the sea that call to him, the features of the sea that give him this "fever." In calling up what seem like memories, he imagines what the sea holds for him in an almost dream-like fantasy. While he does describe a number of literal, tangible components of the ocean—the "tall ship and a star to steer her by," "running tide," and "sea-gulls crying"—what he truly sees is the lifestyle he yearns for.
The speaker desires the "vagrant" and "gypsy" life that to him epitomizes the allure of the sea. In his first line, he describes the sea as "lonely," yet he fills it with numerous wonders. Although he includes people in this list, it is other "fellow rovers" like himself, similarly nomadic, rakish, roguish, alone. This suggests that his real desire is to commune with something larger than people, something larger than society. He hopes to connect with the wild natural world. His vehicle for doing so is the sea, with its lonely vastness being the very thing with which the speaker identifies.
The last line of the poem suggests that even though the speaker is describing a hopeful future for himself, he is indeed in a "sweet dream," looking back wistfully on a past that is over. This idea is reinforced by the repetition of the line segment "I must go down to the seas again" at the beginning of each stanza. He desires still the life he once had, but it lives now only in his mind's eye—as a memory he won't see again.