"This is the best job I ever had."

It's no secret that Jayson -- an All-Star who played 9 years in the league for the 76ers and Nets -- struggled with alcohol abuse for years, leading to tragedy off the court ... before buddies, Curtis Martin, Charles Oakley and Chris Mullin convinced him to enter rehab back in 2016.

Williams got sober (he's going on 30 months now) ... and the experience made such an enormous impact on his life, the 50-year-old founded his own treatment program called Rebound in Jupiter, FL.

"I'm no guru to this. I'm just a guy who's there telling my story, who's surrounded with a great 9-acre, 105-bed facility with the best doctors, the best psychiatrists, psychologists."

The program -- which treats everything from alcohol and drugs to food, sex and gambling addiction -- ain't your prototypical rehab center ... it's an adventure-based curriculum that uses paddle boarding, wave running, hydro boarding, scuba diving and even skydiving to beat addiction.

"We do probably 19 different activities that you have never done in 30 days, and we do that to breakdown barriers, overcome anxiety, build self-esteem and break up the monotony."

Jayson says friends often tell him to leave Rebound behind and "go coach St Johns, get outta that, it's too much, it brings you down" ... but he says they're all wrong.