Final answer:
The "prize" that the speaker says has been won refers to the successful completion of a dangerous and challenging voyage.
Step-by-step explanation:
The speaker in the poem "O Captain! My Captain!" by Walt Whitman uses an extended metaphor to describe the journey of a ship and its captain. The "prize" that the speaker says has been won refers to the successful completion of a dangerous and challenging voyage. It represents the end of the Civil War and the victory of the Union, as the poem was written as an elegy for President Abraham Lincoln after his assassination.