As a Cali local I'd like to clear up a few myths about poison oak. It doesn't spread from the blisters. Most people make the mistake of not knowing they touched some, then scratch all over and it shows up everywhere in the next few days.
The best method I have for avoiding is is #1 never touch any plant if at all possible, early in the year poison oak doesn't have leaves but can still spread just from the stem. If you're on a run that has poison oak, jump in the water after every portage and completly immerse yourself. If you scout something and don't touch any plants but walk through some, get at least waist deep in the water before getting in your boat. An easy way to get it everywhere is get it on your legs, transfer it into your boat...on and on. When you get off the water jump in and dunk your boat too, then shower when you get home. The good news is once you get it all the time you don't get it as bad, my third patch from this year is small and not annoying at all.