The similes in this poem show how much wonder and awe the speaker feels.
The main point of this poem is that the speaker feels awe and wonder when he first looks into this particular work of Homer's. To support this idea, the poet uses two major similes that show this feeling.
First, he talks about feeling like an astronomer who has discovered a new planet. Second, he talks about being the first European to see the Pacific Ocean (it wasn't Cortez, by the way). Both of these similes call to mind the sort of awe that one feels at a new discovery. Both of the people referred to in the similes have in some way had a new world opened up to their eyes. This is the idea that the poet is trying to convey about his reaction to Homer's work.
The similes in lines 9–10 and 11–14 convey the thrill of being the first person to see an unknown planet or an unknown ocean. With that thrill comes the awareness that the universe is larger and more amazing than had been dreamed. Keats has found exactly the right similes for this enlarged sense of the world. The metaphor of a new planet swims suggests that the planet is active, engaged in its own purpose in the universe. The other words, as metaphors or similes, would suggest a more vague, less purposeful mission. The comparison is apt, since the vehicle is the power of Homer, who in The Iliad and The Odyssey was an intentional and deliberate poet.