Zac Efron is having difficulties training to be a lifeguard for the upcoming movie adaptation of the Baywatch TV series, according to the show's star David Chokachi.

The High School Musical actor began shooting the film version of the 1990s show alongside Dwayne 'The Rock' Johnson, who is playing David Hasselhoff's character Mitch Buchannon, earlier this week (beg22Feb16).

Zac, who is playing lifeguard Matt Brody, a part originated by David Charvet, is struggling to get to grips with the ocean, according to David Chokachi, who played Cody Madison in the TV series.

David tells E! News, "Alexandra Paul, who is Stephanie on the show, her husband trains triathletes and I know he's training Zac Efron to be ocean competent. Nothing against Zac, but I heard he's got a little struggle going on with his affiliation with the ocean... He's having a little trouble, let's put it that way."

He defends Zac, admitting learning be a convincing lifeguard on camera is tough and it took him months of training to get it right.

He adds, "People think, 'Oh, we're gonna go swim in the ocean.' But it's different. If you're actually going to portray a lifeguard, there's a lot going on - how you enter the water, how you get to a victim... You gotta be legit or you're gonna look like c**p on TV...

"I'm sure he's struggling because I trained for like three months before we started filming with a real lifeguard, Mike Newman. He kicked my butt. It was hard. The water's freezing, you're running in in your shorts, and you're spending a lot of time in the water, and the waves, you kinda get used to it."

Dwayne posted a picture of him and Zac on the set on Instagram.com on Tuesday (23Feb16) and wrote, "#BAYWATCH Day 1. My character 'Mitch Buchannon' just gave @zacefron's character 'Matt Brody' the all important beach speech about what it means to have manly 'Alpha balls'... World get ready to have some fun, 'cause the beach will never be the same again."

Hasselhoff and Pamela Anderson are in talks for cameo appearances in the movie, which also stars Alexandra Daddario and Priyanka Chopra.