A ship is a common mode of transportation in Dragon Quest game series games. The concept of giving the Hero's party a ship first appeared in II and has appeared in all the subsequent games in the main series.
Gaining access to the use of a ship is typically the result of completing some sort of quest or objective. Normally, the point in the game when a ship is acquired is near the mid-point of the game, as having a ship allows access to areas already visited and new areas which are fundamental to the next portion of the game. In the case of both II and III, acquiring the ship is a segue to quests which requires the Hero to travel all over the world to retrieve specific items.
The ship is given to the Hero/Heroine after bringing the king of Portoga some Black Pepper. After obtaining the Sailors Thighbone, The Hero & party encounter a Phantom Ship which contains the Locket of Love.
In Chapter 5, The Hero/Heroine can use the ship after helping Torneko Taloon relight the sacred fire in the Pharos Beacon. Prior to Torneko's return to Porthtrunnel, a Mini Medal can be found on the ship and can be missed entirely if not obtained at that point.
After persuasion by a Spirit, the elder of Ghent allows the Hero and party to use the ship, but only with Nevan accompanying them. Though until Murdaw's defeat, the ship will only travel to Murdaw's Keep. After obtaining the Mermaid Harp to use for underwater travels, The Hero & party eventually come across a Sunken Ship which contains many treasures, including the Ultimate Key.
There are two ships. You obtain the first one from Prince Keifer immediately before traveling to the village of Rexwood in the present. The second ship belongs to the Marle de Dragonne, and is available in the second disc after the sealing of Estard.
The Hero/Heroine is given a ship from Ms. Marion Bloom, after saving her substitute from her kidnappers.
Traveling in the ship is much like traveling on land. Typically, the ship travels at the same rate of speed as travel by land. The party also will continue to encounter monsters while on the ship, although they will be aquatic rather than land monsters. The ship also follows the Hero whenever the Zoom/Return spell or Chimaera Wing is used. The ship relocates to the nearest body of water to the return point. This keeps the player from having to remember where the ship was last used.