What's the main idea in "O Captain! My Captain"?

It is well known that Whitman's "O Captain! my Captain!" is a lament for the death of Abraham Lincoln. Even without that knowledge, however, the poem remains effective and moving, in large part because the idea and emotions it conveys are not necessarily tied to that singular situation.
The main idea, or point of emphasis, in this poem is underlined through Whitman's use of repetition: the Captain is "fallen cold and dead." Whitman repeats this phrase three times, and it is the final line in the poem and thus the idea retained by the reader. The death of the captain has not been for nothing—after all, the "fearful trip is done," "the prize we sought is won," and the people are "all exulting." So, the tone of the poem, while melancholy, is not without its lighter elements. The captain has died in achieving his goals, and the people recognize that, with "the flag . . . flung" for the fallen captain, while "the bugle trills." In recognition of the captain's sacrifice, there are "bouquets and ribbon'd wreaths," and the shores are "a-crowding."
We know that, for the speaker himself, this "victory" is not yet sufficient to override his sadness over the loss of his captain. He moves "with mournful tread," his heart "bleeding drops of red." However, there is a glimmer of hope at the end of the poem that, in fact, he does recognize that the sacrifice may have been worth it. He encourages the celebration of victory which validates his captain's sacrifice: "Exult O shores, and ring O bells!" So, arguably, the key idea in this poem is that to die for a "dream" is a sacrifice which may ultimately be worth it, if the "ship is anchor'd safe and sound," whatever the sadness suffered for the loss of a "father."

Walt Whitman's poem "O Captain! My Captain!" is an elegy poem, written in tribute to Abraham Lincoln in the year that he died, 1865. It is an extended metaphor and contains three stanzas. The main idea of the poem is that the civil war has ended, which is a cause for celebration, and the reason it is over and won is because of the "ship's captain", Abraham Lincoln, who was instrumental in that war, and now has died without really being able to enjoy the fruits of his labor. 
Using the imagery of a ship, the first stanza talks about the celebration going on because the "prize we sought is won." This is referring to the end of the civil war. At the end of the stanza, the poet laments the death of the captain. 

"But O heart! heart! heart! O the bleeding drops of red,
Where on the deck my Captain lies, Fallen cold and dead."

The second stanza is a plea for the captain to rise up, to join in the celebrations that are for him, and to not be gone from them forever. 
In the third stanza, the poet seems to have accepted that the captain is indeed, dead, and isn't coming back. He is mourning the loss even as those around him celebrate the victory.