On Deso this July, there were afternoon thunderstorms every day. In a steep and narrow section, lightning struck a pinnacle half-way down the all rock slope, maybe 1000' away. Definitly the loudest thing I've ever heard, and I've been close to 16" naval gunfire, and large dynamite charges. We rowed a little faster away from the cloud. I advise turning off all electronic devices and continuing to float. I have had lightning hit very near me on an ocean beach at the water margin, maybe 30 feet away, and on another occasion, it hit an antenna I was standing next to. I've also been in a deep and narrow stream canyon, and seen it bypass the ridges and hit the very bottom. But most of my encounters were on high ridges. It is clear that there are no fool-proof measures to avoid it. At least if you keep floating, you avoid secondary blasts coming off rock or trees. The truly paranoid might try bringing a thick rubber welcome-mat and poncho and try to hover over that and keep it dry.
That said, I was in a nighttime Yampa storm in 2011 that boasted over 150 very close strikes in an hour. We were camped on a sandbar in the river, and many of the lightning strikes on the rim produced rockfall that you heard plunking into the river beside the tent. There were two Christian men on the trip that never slept a wink. I drifted off to Never-land, musing that if it's my time to go, I'll go without regrets.