Nick Cannon -- Naked But Not Afraid of Mariah Carey

Nick Cannon took the first public shot at his estranged wife Mariah Carey ... by putting his soon-to-be single body on display ... for the sake of charity of course.

TMZ broke the story ... the couple's divorce is a "done deal." Nick is clearly telegraphing ... he's taking the split in stride.

Cannon accepted the ALS Ice Bucket Challenge from Spice Girl Mel B and upped the ante by completing the task in the BUFF!

Nick also makes a point of talking about his wiener ... fond memories, Mariah?