Go to the "Watched" sheet
Scroll to the top of the page, and in column J click on the green icon next to "Watched?"
Unselect "Yes" from the list and click "OK"
This will now only display films which you've not marked as watched on this sheet, so be aware and remember to reset the filter if you need to. Please also note that when you mark additional films as watched, you may need to re-do this filter or reload the page.