With full rocker tube pontoons, never anchor. They'll surf and possibly swamp. Flatter/wider hulled pontoons (more the whitewater grade) handle being on anchor quite well. Have anchored my Steelheader in chutes before, and in heavier current fine. If you use an anchor on a pontoon, a pyramid is the only way to go. You want heavy/dense weight. A balled up chain anchor (they do have the drag chain anchors) aren't bad. Still have had the best of luck with pyramids.
if you don't want to swamp your pontoon and lose your gear and possibly your life you should never anchor it, even moderately fast moving water and dragging a chain will certainly get you into trouble. Use it to anchor on the beach.