Russell Simmons -- Stripping Down with New GF ... AND Ex-Wife

Russell Simmons could teach Halle Berry a thing or two about breaking up. He's on such good terms with ex-wife Kimora Lee, he kicked it with her AND new GF Hana Nitsche Wednesday on the beach in St. Barths.

Simmons and Kimora Lee divorced in 2009 ... but they stayed unusually and refreshingly close post-split ... sometimes vacationing together with their respective SOs.

There's no question Russell can pull some impressive ass -- but we gotta ask ...