You should have waded in right next to him, given him a little pinch on the butt, and asked him if he was looking for a boyfriend. More than likely it would spook him and he would take off, only problem would be if he said yes, then you'd have to figure a way out.
But seriously, you'll run into jackasses out there whether it's a river or lake, best thing to do, as you did, is just walk off and let them have their fun.