To complete a watch in the app:
Click 'View' on the watch notification at the bottom of your calendar
You can also select the day in the calendar and then select 'Watch' from the day selector panel at the bottom of the page
Open the duties checklist by tapping on the down arrow next to the checklist name
Tap on the tasks to complete them
A percentage complete notification can be seen at the bottom of the screen. Once at 100%, the duties checklist is complete
A watch will be completed once the end time of the watch has been reached and the duties checklist (if there is one) has been completed.