Day One by Bloom Built, my go-to journaling app for iOS and the Mac, has had an app for the Apple Watch since April 2015, but it used to be hampered by the limitations of watchOS.
With watchOS 2, apps can now run directly on the wearable device and more independently of its connected iPhone, resulting in better responsiveness and faster execution. A brand new release of Day One now includes a native watchOS 2 app, support for 3D Touch Home screen shortcuts on the iPhone 6s and iPhone 6s Plus and more.