I think the insulating value of an inch of rubber is irrelevant when you consider the voltages involved in lightning. After all, air is a pretty good insulator, yet lightning manages to ionize it and travel--what--a mile to the ground. A quarter inch of rubber won't save you. If it would, no one would ever be killed by lightning as long as they were wearing rubber soled shoes...which is pretty common in this day and age.
Lightning does want to get to ground sooner rather than later, and will probably hit a peak or a tree before it goes to the bottom of a canyon to hit the water. So I generally feel safer in a deep river canyon. But there are no guarantees.
The one thing I always think about is, not everyone that is hit by lightning dies...but they are usually knocked unconscious for a while. I'd rather not be knocked unconscious while in a boat in moving water. So if the canyon and river is wide, it may be at least a little safer to be on shore, but as low as you can get.