A ship is a common mode of transportation in Dragon Quest games. The concept of giving the Hero's party a ship first appeared in Dragon Quest II and has appeared in all the subsequent games in the main series.
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 (or Return) spell or Wyvern/Chimeara 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 (or worse, having it stranded on a remote island).
Access to 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 Dragon Quest II and Dragon Quest III, acquiring the ship is a segue to quests which requires the Hero to travel all over the world to retrieve specific items.
In Chapter 5, The Hero can later 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 is possible to miss entirely if not obtained at that point.
After persuassion 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.
The Hero and his party discovers a landlocked ship in the desert south of Trodain. They enlist the aid of Ishmari, a magical poet who can invoke memories (in this case, the ancient sea), but when his harp unexpectedly breaks, the party must first acquire a replacement harp from the Land of the Moles that was stolen from Ascantha's treasury.
The Hero is given a ship from Marionette after saving her from kidnappers.
Ship on Pilchard Bay from Dragon Quest VII