Ship

Whitman’s uses the ship in “O Captain! My Captain!” to represent the voyage of the United States as a whole. We learn that this was no pleasure cruise, either. The ship stands in for the nation as it moved through the difficult times of the Civil War ("our fearful trip"), toward a peaceful solution (even if it was one not entirely agreed upon, as evidenced by Lincoln's assassination). As a result of the difficult voyage (the Civil War), the ship looks “grim and daring.” The imagery of the worn vessel parallels the condition of the people who were affected by the war. Many citizens, soldiers, and leaders suffered starvation, poverty, and anxiety over its course.

• Line 2: The ship that represents America has “weather’d,” or weathered (as you probably figured out), rough storm winds while out at sea. A weathered ship like this is going to have tattered ropes and sails (the steam-powered ship had just come into use in the mid-1800s). The wood is going to be rotted and discolored. Think of a ghost ship from pirate movies. This ship has seen some tough going.
• Line 4: “Keel” has multiple meanings. The first is the part of the ship that goes along the underside that gives the ship stability in the water. This usage really doesn’t make much sense, though, because one can’t see a “steady keel” unless the boat has fallen over. The other use of keel here is as a synecdoche. The keel, a part of a boat, is used to represent the whole boat. More importantly, "the steady keel" lets us know that, despite its rough trip, the ship (like the nation) remains steadfast and true. 
• Lines 7 and 15: The deck is the top part of a ship’s body. It’s what the people (including our speaker) walk on. Except the captain. He lies on the deck, since he's dead. That he's on the deck of the ship, though, is significant. It's not like the captain jumped overboard, or died below in one of the storage rooms. He died at his post, from a vantage of command. Much like Lincoln was gunned down while still the president, this captain has died on the deck. We're invited to make that connection between his position on the ship and Lincoln's position in office. 
• Line 19: The ship comes safely home, and so it is a "victor," or victorious, ship (not this kind of victor). The ship’s safe arrival here represents the nation’s safe return to normal life after the Civil War. Despite past hardships, the ship—and the country—are still afloat.