Boult (3-22), who played for Mumbai between 2020-21, dismissed Rohit Sharma, Naman Dhir and Dewald Brevis for golden ducks in his first eight balls on Monday.
The bowler struck twice in the first over, swinging the ball away from Rohit to coax the Mumbai opener into edging behind before trapping Dhir lbw on the very next delivery.
Boult has now taken 25 wickets in the first over of an innings in the IPL since 2020.
"Trent Boult is just so accurate," Watson said on Jio Cinema. "He rarely misses his execution with the brand new ball in the first over. He hits the ground, sprinting, so accurate.
"Ball swinging back in, he is just an absolute weapon and he continues to do it over and over again. It's like he is just getting young with age.
"He is so energetic and he is always on the money from ball one and what an asset to have for the Rajasthan Royals."
Boult said he was glad to see the ball swinging at Mumbai's Wankhede Stadium, adding: "It was a little different from our beautiful palace back in Jaipur.
"It was not just me, it was a collective effort. It was good to win the toss, and then you have to put the ball in the right areas and get some early wickets. It's a recipe to succeed."
Rajasthan, who top the IPL standings with three wins from three matches, next face Royal Challengers Bangalore on Saturday.
