As opposed to Skinny Dipping, which is often reknowned for being quite visually pleasing to onlookers, Fatty Dipping is the act of one or more overweight individuals creating a public eyesore by bathing nude outdoors in a (very) large body of water.
Think Free Willy and Friends.
Othon : "Oh my fucking god, lets get the hell out of here"
Liz : "Why?"
Othon : (points to a congregation of naked fat people frolicking about under a waterfall)
Liz : " Holy shit! How the hell did I not see THAT? Fatty Dipping ought to be made illegal!"
Othon : "I know right. My eyes bleed profusely. Somebody call Green Peace and have them dragged back out into the ocean".