Day One is available on Apple Watch. As of version 2.6 on iOS, we now have a complication available in the Modular watch face.
Tap the complication or open Day One from the applications view. Scroll through your different journals using the Digital Crown.
Tap the Microphone icon to dictate an entry. Tap the Location Pin to access recent locations. Tap any location to create an entry in Day One with that location.