The 43-year-old actor and 28-year-old High School Musical star posed for photos in their red and blue lifeguard outfits while filming the big screen adaptation.
"#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.' Zac's ad-libbed responses were brilliant! Talented and great dude," Johnson wrote.
"World get ready to have some fun,' cause the beach will never be the same again. #OnSet #Baywatch #Day1 #AlphaBalls #NowWhoNeedsMouthToMouth?" he added.
"First day protecting the bay is a wrap. Thanks to the big guy (Dwayne). Stay tuned. More adventures lie ahead. #speedoready #baywatch," Efron captioned his own post.
The Baywatch reboot will co-star Alexandra Daddario, Kelly Rohrbach and Priyanka Chopra, with Ilfenesh Hadera as Johnson's love interest, Stef. The actor also shared a photo with Hadera, who plays Deb Kawi on Showtime series Billions.
"Our 5'11 talented, bad [expletive] smokestack @ilfenator dominating the role of 'Stephanie Holden' and some big bald, inked up brown dude giving the gangsta point. #OnSet #Baywatch #BaeWatch," Johnson captioned the snapshot.
The original Baywatch series ran from 1989 to 1990 on NBC and from 1991 to 2001 in syndication. The movie is directed by Seth Gordon. It opens in theaters May 19, 2017.