Despite the fact that here are many answers, none of them provides an understanding of how the folder structure of the iOS 8.3 simulators have changed and are not providing a quick way to find the App's Data (Documents folder).
Since iOS 8 the Data storage folder(s) of an App are separate from the App's executable files while the iOS 7 and below are having the same folder structure, the only difference being the fact that all the simulators (different types and versions) are now in one, big folder.
So, the path to an iOS 8,7,6 simulator is the following:
~/Library/Developer/CoreSimulator/Devices
Every simulator is now contained in a folder named with an unique identifier which changes at every reset of a simulator.
You can find the Identifier for each of your devices & simulators by going to Xcode > Window > Devices (the first 3 or 4 characters of the identifier are more than enough to remember).
To find the one on which you have installed your app(s) on, take a look at your Run scheme > devices (screen 2).
Now, after you identify your simulator, depending on its version the folder structure is very different:
On iOS 8 the executable and the data folder of an app are in different folders:
Executable:
~/Library/Developer/CoreSimulator/Devices/[simID]/data/Containers/Bundle/Application/[appID]
Data Folder:
~/Library/Developer/CoreSimulator/Devices/[simID]/data/Containers/Data/Application/[appID]/
Documents Folder:
~/Library/Developer/CoreSimulator/Devices/[simID]/data/Containers/Data/Application/[appID]/Documents
On iOS 7 and below the folder structure is the same as before only remember that now every simulator is in the same folder (see above).