I would recommend a water dish, for 3 reasons that I can think of offhand. One, it's an insurance policy against the substrate getting dry - if, for example, you get called out of town unexpectedly and nobody mists the tank for a week. Life happens. Two, it's a convenient way to feed some varied foods, such as blackworms or cut-up earthworms. Three, it give the animals a way to "tell you" when they are interested in becoming aquatic - you see them spending a lot of time in the water dish.