The tone of the poem is a happy, but sad and depressing at the same time. The speaker in the story is depressed at the fact that his captain died.
"But O heart! heart! heart! O the bleeding drops of red, Where on the deck my Captain lies, Fallen cold and dead."
"The port is near, the bells I hear,"
Each stanza that has eight lines is split into two parts. The first four lines are long and talk about Lincoln's achievements. The last four lines are short and talk about Lincoln assassination.
The theme is to always honor people who've helped our nation, even if they have done something little or big.
The entire poem is an extended metaphor. The ship is being compared to America, the captain is being compared to Abraham Lincoln, the captain dying is compared to Lincoln's death, the prize that they've won is the civil war.