The 29-year-old actor had to shape up for his role as a lifeguard in the hotly-anticipated movie and was incredibly dedicated to the cause, according to his trainer Patrick Murphy.
Zac said: "Patrick's training was confusing at first...I just thought I was showing up to lift weights and get a deeper six pack, which is hard enough already.
"But rather than the typical bodybuilding type of movements I had become accustomed to, Patrick pushed me to my 'functional' limits with a very multifaceted style: strength, explosiveness, fast paced, calculated, diverse and gut-wrenching workouts. It was fun ... you never know what he's going to throw at you the next day."
As well as one-hour gym sessions, Zac also underwent professional lifeguard training.
Patrick told E! News: "There are no shortcuts. Zac got so super lean that he only looked bigger, but he only got a little bit bigger. You can see deep lines and muscles popping because he's so lean. He's Bruce Lee-lean ...
"[They did] 20 alternating jump lunges in one place, 20 jump squats, 60 mountain climbers, power pushups, followed by a run up five flights of stairs, then one-leg squat hops using a TRX rope ...
"You never know what comes next. Create change to continue to create results. They would laugh looking back at the first week of training - eight weeks later, it was a whole other ballgame."
Zac had to slip into high heels for a scene in 'Baywatch' and joked he had "mad respect" for women after donning those shoes.
Taking to Twitter, he wrote at the time: "Mad respect to all the ladies who can handle heels. 1 of my hardest stunts EVER on #Baywatch (sic)"