“My tight-fitting 100% cotton-free boxer-briefs keep the chaffing in other sensitive areas at bay.”

Some more ideas to stir in the stew:
I’ve had best results in poly-cotton boxer briefs. And you’re right; keep it ‘tight’--- like biking shorts.

Also, the back sweat will roll down and collect there. The salt water will turn your skin raw from the rubbing. So if you could wick (or other measures to keep it cool) off your back sweat before it ‘rolls’ that should help.

-Barry