STOP THE PRESSES: Boozehound GIBSON parties hearty with pals on…WATER!?

What’s wrong with this picture? MEL GIBSON partying totally sober – yet acting like he was ready to slug someone.

Attending a private party at a friend’s Hollywood Hills home, the alcohol-loving actor spent the entire evening drinking water while other guests guzzled away.

My SpyWitness reveals: “Mel was adamant that he wasn’t drinking anymore, and he was getting really PO’d when people kept asking if he wanted to trade his water for something stronger.”

At first he simply replied he was fine, but as the night progressed and some of his so-called pals tried to entice the recovering stupor-star to drink, he got really upset.

More than a dozen times, Hollywood’s hottest hothead held up his hand, turning down any type of alcoholic beverage.

“How many times do I have to say, I’m NOT drinking!?” he finally fumed – then put down his water and split.