In a televised speech on the House floor about his arrest in Connecticut last Friday on driving-under-the-influence and marijuana-possession charges, House Minority Leader Robert A. Watson admitted to using marijuana to treat flare-ups of the pancreatitis that landed him in the hospital last November. Watson, R-East Greenwich, said he took a small amount of the drug with him when he went to Connecticut that day to help a friend move because he had had a pancreatic attack the day before, and wanted the drug handy if he had another severe one.