The larger the water bowl, the more humid the environment will be based on surface area. If you are having trouble keeping the humidity up, you can partially cover the vents to decrease water loss from the tank, you can mist the tank periodically, or you can provide a moist hide for them to retire to if they are feeling the need for increased humidity.
Personally, my snakes use their water bowls to regulate their humidity. If they are feeling dry or are approaching shed, they just take a periodic soak in the water dish.
Rather than humidify the entire tank and worry about going overboard, causing increased fungal growth, bacterial growth, and the like, I'd just provide a humidity box for them to utilize when they need it. A box with a hole twice the diameter of the snake filled 2/3 full of moist vermiculite, moss, paper towels, or whatever will do it. Just remember that moss and paper towels will grow mold and need to be changed fairly frequently.