The ex-NFL superstar -- who's set to square off with MMA fighter Brian Maxwell on the undercard of the Mayweather-Paul fight on June 6 -- broke it all down for us this week ... explaining he's sparring Jermell and Jermall Charlo to get ready for his match.

And, he says he's taking a beating from the boxing stars every time ... telling us of a recent sparring sesh, "I was f***ing swimming."

"Third round," Johnson added, "I was in the middle of the ocean with no life vest and I was f***ing drowning."

Not familiar with the Charlos? They're 2 of the best boxers in the world ... and have a combined 65 professional wins with only 1 (very controversial) loss.

But, the 43-year-old -- who said he's also training with some of the Charlos' sparring partners -- explained he's hopeful it'll all help when the lights come on against Maxwell in Miami next month.

"I've been training with the Charlo twins so I'm here now and it's funny I just finished sparring one of [their] sparring partners and it was fun," Johnson said. "It was a great experience."

"It was an experience where I was taken advantage of and it's a great thing. Obviously going in the ring with someone that does it at the highest level and learning the mistakes. I like it like that."

Johnson continued, "There was no other way to prepare for something of this magnitude. I had to come here. It's just -- there's no other way."

Johnson wouldn't guarantee a knockout over Maxwell -- even despite the training efforts -- but he did tell us it's going to be a show nonetheless.