EFF:

If you want to keep Google from combining your Web History with the data they have gathered about you in their other products, such as YouTube or Google Plus, you may want to remove all items from your Web History and stop your Web History from being recorded in the future.Note that disabling Web History in your Google account will not prevent Google from gathering and storing this information and using it for internal purposes. It also does not change the fact that any information gathered and stored by Google could be sought by law enforcement.

Hat tip to Daring Fireball.