What does the Hispaniola signify in the novel Treasure Island?

The Hispaniola is the ship used by the characters in Treasure Island. As a symbol, it is multi-faceted, meaning that it symbolizes different things depending upon the dramatic needs of the scene.
Firstly, the ship delivers Jim on his adventure. It is described as a fine, fast, and easily-controlled vessel, and therefore the perfect ship to take on such a quest. However, when Jim realizes that the crew are not trustworthy, the Hispaniola becomes like a prison on the ocean. Because they are far from land, there is no escaping from it or the dangerous pirates who come to command it. And, as the other answer addresses, the ship becomes a symbol of hope when the characters are rescued by it.
Taken altogether, the Hispaniola is a symbol of adventure and all that comes with it. While adventures can be exciting and a gateway into new worlds, they can also be packed with peril—as Jim comes to learn.

The Hispaniola is the ship that takes Jim Hawkins, his companions, and the pirates to Skeleton Island in search of the treasure. Simply put, it is a vessel for transportation. However, more figuratively, it represents the themes of hope and betrayal seen in the novel.
We first hear of the ship from Trelawney, who writes to the Doctor: ''The ship is bought and fitted. She lies at anchor, ready for sea. You never imagined a sweeter schooner—a child might sail her—two hundred tons; name, Hispaniola.'' At the time the novel was written, schooners were some of the fastest and nimblest of ships available. Jim and his companions look to the Hispaniola as the means that will allow them to achieve their goal, finding the treasure. They look to the ship full of promise and potential.
However, once at sea Jim finds the Hispaniola to be a trap. Not long after setting out, Captain Smollet expresses doubts as to the trustworthiness of his crew, who indeed turn out to be murderous pirates. The crew, who we are introduced to aboard the ship, are dangerous and unpredictable. Throughout the brief description of the voyage, it is hard to distinguish the language used for the crew or the ship, combining them into one large notion of uncertainty and potential betrayal. Indeed, it is on the ship that a terrifying fight takes place between Jim and a pirate.
In its role as a symbol of hope, the Hispaniola comes into play again near the end of the novel. It acts as the rescue vehicle of the protagonists and conveys them and their treasure to safety after leaving the last remaining pirates stranded on the island.