He’s been a plumber for 40+ years, he will know what it would have cost you, if you tell him.

You can be matter-of-fact about it and not belabour the point by catastrophizing a possible outcome.

“Hey dad, we dodged a bullet. A fitting was loose but they caught the leak and there is no permanent damage, I’m going to go fix it”

If he’s not an idiot he won’t need reprimand, he’ll know he fucked up, and the potential size of the fuckup.