The former Aston Villa striker netted in the 40th minute to give Al Hilal the lead, calmly steering the ball home while lying on the turf after being upended in the penalty area by Agustin Soria.
"I felt like I worked hard for the team, I got my goal, and that's what I'm here to do: get goals for the team and ultimately work hard and put everything into the game," said Watkins.
The win followed Al Hilal's opening victory over Al Gharafa in the Western League phase of the competition last month and was sealed when Ruben Neves scored from the penalty spot in the 65th minute.
Al Hilal were joined on six points by Shabab Al Ahli after the side from the United Arab Emirates beat former champions Al Ittihad 2-1 in Jeddah.
Sardar Azmoun struck twice in the opening 16 minutes, converting from close range in the seventh before the Iran international ran from just inside the Al Ittihad half before finishing.
Richard Rios pulled one back for the hosts in the 66th minute, but Shabab Al Ahli held on to take the points.
"We were very focused, and we believed we can win this game," said Azmoun. "We analysed them well, and we worked on the pitch very hard, and that's why we won the game."
Al Ain from the UAE and Iran's Esteghlal also have perfect records after two rounds of the eight-match league phase. The top eight finishers in both the west and east regions advance to the last 16.