I never use water when making jerky...or anything else for that matter. I have gone to using sand on every cook, especially my jerky. That goes against what you are trying to accomplish..DRYING the meat.
I control all my fires by the amount of lit coals I add to the unlit ones. By adding just a few lit coals you can easily keep the WSM between 150? - 180? for many hours. Just make sure to line the water pan....makes for easier clean-up.
Not sure if you have seen my jerky page so here is a link....