Divinity Original Sin: Fire Fire Fire

Fire! Fire! Fire!

Fire! Fire! Fire! is a side quest in Divinity: Original Sin

It is one of the first side quests you can do as you enter the Cyseal Harbor

Journal

A ship has been set on fire by the orcs in the harbour!

When you enter the Cyseal Harbor, you will see some NPCs running asking for help, screaming that "Our ships on fire come help"
Go to the docks to the southwest and locate the Ship that's on fire. See map below.

Use a Rain Scroll on the Burning Ship.
You might have found a Rain Scroll at the beach as well as another one at the Tutorial Dungeon.
Alternatively You could try bringing the Water Barrel close to the ship and throwing it at the ship, or fill the Bucket with water and help the other sailors who are putting out the fire.
If you are carrying a Water Arrow you could shoot that at the fire
You have a very limited time to extinguish the fire. If you do not extinguish the flames on time, the ship will burn down and the quest will fail.

Using a Rain Scroll is the fastest and easiest way to complete this quest.
You should go to the Burning Ship as soon as you enter the harbor in case it is already burning otherwise the timer will run out and quest will fail.