If Watergate never happend, or if it happens and Nixon doesn't cave into Haldeman's demands to issue a cover up, and comes out with a clean image, then his legacy is one for the history books. He will go down as a great Foreign Policy President and a decent Domestic Policy president.
Ford still get's the Veep in 1973, and runs in 1976, winning w/out the Pardon over his head. He is re-elected in 1980 and retires after inauguration day 1985. There would be no Carter or Reagan if Dick was without Watergate. The GOP would remain much like it did for years with the John Birch Society still looked upon as the crazies of the party and the Nixonian, Ike and Ford Repulicans remaining the establishment.
It would pretty much look like this IMO:
1969-1977: Richard Nixon (R), 37th President
1977-1985: Gerald R. Ford (R), 38th President
1985-1993: Dick Gephardt (D), 39th President
1993-2001: Dale Bumpers (D), 40th President
2001-2009: John McCain (R), 41st President
With the possibilty of George H.W. Bush and Governor Tommy Thompson somewhere in there. G.H.W. Bush ran as a Moderate Republican when he challenged Reagan for the Nomination in 1980.