"When the tide goes out you'll see whose been skinny-dipping" meaning, don't lie or cheat because eventually you'll be caught.
My father has always had a couple of colorful ones, I guess.. I'm not sure I can repeat them here, but I"ll try.. *L*He'd say someone was so dumb they couldn't pour water* out of a boot if the instructions were written on the heel.
He could not find his way out of a paper bag with a detailed map and a flashlight.