How do I clear my YouTube watch history, and stop it recommending embarrassing stuff?

Go to your profile picture > Settings > Manage all history (it opens your Google account's YouTube History page), or on desktop use the left menu > History > 'Clear all watch history' on the right. To stop it tracking, hit 'Pause watch history' in the same place. Clearing plus pausing is what actually resets recommendations. You can also remove individual videos: on the History page hover a video and click the X to delete just that one, which fixes recommendations without nuking everything.

Deleting one bad video with the X is the pro move, it removes just that from your history so recommendations recover without losing your whole watch record.

Pause watch history if you want to browse without it affecting recommendations at all. Great for researching random stuff you don't want haunting your feed.