But O heart! heart! heart! (5)
The speaker is so overwhelmed that he can’t express the pain he’s feeling at the sight of his fallen captain. That's some serious suffering.
This arm beneath your head; (14)
As a nurse during the Civil War, Whitman likely held many dying men. Not fun. The pain he must have experienced in such a moment really makes this image—the dead captain in the sailor’s arms—the climax of the poem.
It is some dream that on the deck (15)
Again, the sailor cannot face the reality of the captain’s death. The suffering is unreal, in a way.