Since I use a rack system with tubs, my snakes have no reason to soak in their water bowls. I use small water dishes (maybe 3-4 inches in diameter) for them to drink out of. I fill the water about an inch and refresh once daily (unless they poop/pee in it). There really is no reason for it to be a specific depth, though in your case since you have a tank, humidity issues might cause the ball python to need to soak. Choose a bowl that is going to be large enough for the snake to curl up in and deep enough to submerge at least part of its body.