In this tutorial you can learn how to build docks for ships to load and unload and how to pick a suitable location for a dock. Ships can for instance be used to haul oil from an Oil Rig to an Oil Refinery. Once you get the hang of building ship routes, you can use ships for other things as well.
Finding a suitable location
First, you'll need to find an oil rig and an oil refinery to which you want to bring the oil from the rig. The good thing about oil rigs is that they have a built-in dock so you don't need to construct one there. The only dock you need is at the oil refinery. Refineries are only found near the coast.
Found an oil refinery close to the waterside? Good.
Building a dock
3. Move the mouse along the edge of the water: the dock needs to be placed on a sloped tile that is connected to a water tile.
4. Keep an eye on the Dock building window and make sure that Oil is displayed after Accepts before building the dock.
5. If the cursor displayes two white squares and oil is accepted at the location you picked, just click with the left mouse button to place the dock (possibly you'll have to terraform a bit first in order to create a suitable location).
And that's all there is to building a dock! In order to buy a ship to transport the oil, you'll have to construct a ship depot first.