Zac Efron & Kelly Rohrbach Breast Rescue Ever

For once, Zac Efron and The Rock's bulging muscles were NOT front and center on the set of "Baywatch" ... 'cause it was time for Kelly Rohrbach to shine.

Yeah, Zac's in the shot too -- but Kelly's the one providing the flotation during the rescue scene they shot Thursday in Georgia.

Much like the kid they're saving, Kelly's swimsuit is holding on for dear life.