Firstly, gathering evidence is not protected by absolute immunity, per the decision:
> By contrast, when prosecutors have sought evidence from the President, we have consistently rejected Presidential claims of absolute immunity.
Watergate would not have necessarily fallen under an "official" action if the prosecutors can successfully argue that it was done in personal interest or towards the political campaign. And congress is still free to impeach a president for the official acts.
> Presidents cannot be indicted based on conduct for which they are immune from prosecution. On remand, the District Court must carefully analyze the indictment’s remaining allegations to determine whether they too involve conduct for which a President must be immune from prosecution. And the parties and the District Court must ensure that sufficient allegations support the indictment’s charges without such conduct. Testimony or private records of the President or his advisers probing such conduct may not be admitted as evidence
Emphasis mine. There's quite a lot in Watergate that would be changed by this decision, it seems.
A counter example is that we have recordings of JFK planning and discussing the legality of Operation Northwoods, but simply discussing or planning a potential crime is not a crime in itself.
https://thehill.com/regulation/court-battles/4750581-supreme...
> I think that Nixon would have survived. I think he would have walked under this ruling
> Virtually all of his Watergate-related conduct and virtually all that evidence falls in what could easily be described as official conduct.
> “I’m not even sure you could get to the Nixon tapes under this ruling,” Dean speculated. “What its implications are with the existing Supreme Court ruling, in U.S. v. Nixon, is a question I haven’t unraveled yet.”
https://www.yahoo.com/news/richard-nixon-had-pass-john-20323...
A pretty bleak day.