Why is this the answer?

The answer is ship because this wonderful riddle uses clever word pictures to describe what happens when you travel across the water. A riddle is like a secret code, and we need to unlock the two main clues to solve it. The very first part says, "Whilst I was engaged in sitting." This clue tells us that the person telling the riddle is resting or being still. Imagine you are a passenger on a giant wooden boat, like a grand sailing ship from a movie or a big ferry. You are sitting down comfortably on a bench or maybe even in a deck chair, watching the waves roll by while the ship moves. The person who is doing the seeing and telling the story in the riddle is simply a passenger resting on the ship, not the ship itself. Now we can move on to the most wonderful and tricky part: "I spied the dead carrying the living." This is the secret language of the riddle, which uses words in a surprising way. We need to figure out who "the living" are and who "the dead" are. The "living" are the easiest part to figure out. They are all the people on the boat. This includes the sailors who are working, the captain steering the ship, and the passengers, like you, who are alive and moving around on the decks. The riddle calls the ship itself "the dead." This is because almost every ship, especially when this riddle was first told a very long time ago, was built from huge pieces of strong, thick wood. This wood came from big trees that were cut down. Once the tree is cut and shaped into the parts of a ship, that wood is no longer a living part of nature, so the riddle calls it "the dead." This strong, non-living structure, which is "the dead," is the object that holds up all the busy people, which are "the living," and carries them safely over the wide ocean. The wooden ship is carrying the people on their journey across the waves. That is why the ship is the perfect answer to this amazing puzzle.