What impact did Watergate have on the power of the presidency?
Basically, the Watergate scandal made Americans more distrustful of presidential power. This attitude was reflected in the passage of two laws that both worked to reduce the power of the presidency.
One of these laws had to do with the president's power to run the government. Pres. Nixon had "impounded" (refused to spend) money that Congress had appropriated for various programs and agencies. In 1974, Congress passed a law making it illegal for a president to do this. They did so partly because of a growing concern over presidential power brought on by, among other things, the Watergate scandal.
A second of these laws was the War Powers Resolution of 1973. This law was meant to reduce the power of the president to commit troops to combat without a declaration of war. Here, again, a major cause was general distrust of presidential power. This was caused partly by Watergate.
Watergate led to a general drop in people's trust of presidents. It made them want curbs on presidential power. These things were manifested in Congressional actions such as those listed above.