Well, the clear answer here is that it ended Nixon's career as a political figure. It ended his presidency 2 years before it was going to end anyway, but it also pretty much ruined his reputation and his ability to act as any sort of an "elder statesman" after his presidency ended.
Had it not been for Watergate, I think Nixon would have had a much different career after he left office. He would have been seen as the guy who opened up diplomatic relations with China and who got the Vietnam War ended. I think that he would have been able to have a career more like that of Jimmy Carter who has become a major player in various international affairs after he left office.
The Watergate Scandal has defined Richard Nixon's legacy as a former President. Despite the fact that he was elected twice, served six full years and successfully extracted us from an unpopular, protracted and expensive war, he will always be remembered for his spectacularly bad judgement regarding this scandal.
Not that I feel this is undeserved, it was a criminal act by a President who had come to believe that he was above the law. He deserved impeachment had he not resigned, but as a historian, I have to acknowledge there were other important and positive things he did during his Presidency.
The Clean Air and Water Acts were passed on his watch, and he aggressively pushed for integration of schools through busing, which was unpopular at the time, even if socially necessary. He successfully drove a wedge between two adversaries, the Soviet Union and China through detente, and locally, where I live, he gave Mt. Adams to the Yakama Indian tribe as sacred native land.
Despite all of this and anything else he may have achieved, he will always be, first and foremost, associated with the Watergate Scandal and the other unconstitutional acts that came before and after it.