In case you ever need to read the console output of an Apple Watch (we can have many connected to one phone since WatchOS 2.2!) connected to an iPhone, e.g. because your Watch App crashes on startup in a production version and you have no idea why, that's an easy job to do.
Connect the iPhone to your Mac using a lightning USB cable, and open up Xcode. Click Window → Devices and select your iPhone from the list of connected devices on the left in the window that opens.
Make sure you have expanded the log pane by clicking the small button in the bottom left corner, as highlighted in the above image. This will show up a pane with tabs for your iPhone's console logs and those for the connected Apple Watches.