Interestingly, John Keats, at twenty-one, could not read Greek and was probably acquainted with Homer's Iliad and Odyssey only from having read the translations of Alexander Pope, which apparently seemed prosy and stilted to him. However, after he and a friend found a more vigorous translation by the Elizabethan poet George Chapman, Keats was enthralled and he and his friend stayed up late to read aloud this work to each other. Toward morning Keats wrote the sonnet "On First Looking into Chapman's Homer" before going to bed.
Of note, too, is the allusion to Cortez, since Balboa, not Cortez, discovered the Pacific, as previously mentioned. Nevertheless, this error does not detract from the value of Keats's poem. Another allusion is to Apollo, the god to whom the Greeks always turned for wisdom. He was the god of prophecy and healing; in Oedipus Rex, Apollo is the god whom the seer Teiresias consults at Delphi. In the last line, Darien is alluded to; this is an ancient name for the Isthmus of Panama. And, of course, Keats refers to Chapman, whose translation inspired him.
There are a couple of allusions that I see in this poem.
The first set of allusions refers back to the times and myths of the ancient Greeks. You have the line about going around in the western Islands. This can be read as an allusion to the voyages of Odysseus. Then there is the the allusion to Homer himself.
The second allusion is to history -- specifically the history of the Spanish "exploration" of the Americas. Here, the poet makes a mistake as it was Balboa and not Cortez who first set eyes on the Pacific Ocean.