Occasionally.  We usually go in June, so how long I swim (and whether I do) depends upon the weather and the location.  Whether I wear anything depends upon if there appears to be someone around nearby.  Don't take a bathing suit, so it is either underwear or not.  Always swim from a campsite, and check out the situation carefully first.  Spartan1 doesn't care much for swimming and I usually can't get him to come in with me, so he serves as "lookout".