The Voyage of the Dawn Treader by C. S. Lewis

The three kids land in the water near the pictured craft and are taken aboard. The ship is the Dawn Treader, and belongs to Caspian X, King of Narnia, who the Pevensie children helped gain the throne from his uncle, Miraz.

Caspian is trying to find the seven lost Lords of Narnia. Reepicheep is also on the ship, and he is trying to find Aslan’s country beyond the seas.

Their first stop is in Lone Islands, where slave trade flourishes, despite Narnian laws declaring that it is prohibited. A slave trader captures Caspian, Edmund, Lucy, Eustace and Reepicheep and a man buys Caspian before they reach the slave market. This man is the first Lord they were looking for, Lord Bern. He moved to the island and married a local woman after being banished from Narnia by Miraz. After Caspian revealing who he was. Bern acknowledges Caspian as King.
Caspian asks Bern regarding Governor Gumpas. Bern reveals that, out loud, the Governor recognizes the King of Narnia as his superior, but, in practice, he would more likely be threatening to Caspian

and Caspian replaces Gumpas, the current governor, with Lord Bern. (mid 4)
At their second stop, the group settles at a different island for a while.
Eustace leaves the group to avoid to work, and hides in a dead dragon’s cave when it starts to rain.

The cave is full of gold and jewels, and Eustace gets greedy. He fills his pockets with the treasure. Waiting for the rain to stop he sleeps and he is mutated into a dragon.

While he is a dragon, he figures out how bad his behavior was and uses his strength to make amends.
Aslan changes Eustace back into a boy, now much nicer than before. Caspian recognizes one of the jewels Eustace had taken as a bracelet belonging to the second Lord, Lord Octesian. They think that the dragon killed Octesian, or Octesian was the dragon himself.
Next, they stop at Burnt Island and then at Deathwater Island, where they find another missing Lord, Lord Restimar, transformed into a statue of gold at the bottom of a pool of water that turns everything immersed in it into gold.

Next stop is at Duffers’ Island (11) and then at the Dark Island, this island being permanently hidden in darkness, and where the dreams come true. On this island, they rescue Lord Rhoop.

Mid-Point

Finally, they reach the Island of the Star, where they find the last three Lords in an enchanted sleep. A fallen star named Ramandu, who lives on the island, tells them that to break the spell they have to sail to the edge of the world and they have to leave one member of the crew behind.
When sailing towards the edge of the world, they pass into an area where merpeople dwell and the waters are sweet.
The waters become so shallow that the ship cannot continue, and Caspian wants to leave the ship and go in an adventure with Reepicheep.

After an argument, the crew does not allow that.

Aslan instructs Caspian saying that only Lucy, Edmund, Eustace and Reepicheep should proceed in the boat.

Fulfilling Ramandu’s tale, Reepicheep goes into a waterfall and disappear, probably reaching Aslan’s country. Edmund, Eustace and Lucy find a lamb, who transform into Aslan and tells them that Edmund and Lucy shall not return to Narnia, and sends the children home, where everyone notice how Eustace has changed.