RedactedNarrative: Operator stated he put the drain plug in before launching. They sat on the lake for a while and when they went to move, the boat felt sluggish. He turned on the bilge while they stopped and tied on a tube to the back of the boat. The Operator stated that when he went to take off, the boat sank in the stern and water came in and then the boat rolled over. All were wearing life jackets. After the boat was righted and the water drained, the drain plug was not in the boat, but still sitting on its tether. There were no injuries and alcohol was not a factor.