I have googled a lot on this, cannot find anything in the Amazon site, or app, that shows movies I have watched.
- log in to your account
- click the Your Account link on the top bar
- on the resulting page scroll down to Personalization
- click on Improve Your Recommendations
- then on the left menu click on Videos You've Watched
If you are an Amazon Prime member:
- Click the "Your Account" button
- Click "Prime Video settings"
- Click "Watch History"