The depth of the dish itself could be quite intimidating. Our adult male won't go anywhere near a dish he feels is too deep. I bought him a much shallower dish, and my baby has a very shallow dish as well, just enough for her to lay in if she wanted to, but it only comes up to her belly.
The water goes on the cold side of the tank to prevent a massive evaporation from the heat on the warm side, this would increase the humidity, which is bad for a desert lizard. My baby doesn't actually use the dish at all for hydration, instead, she uses the humid hide, which is important for shedding skin. This could be as simple as a tupperware dish with damp papertowels in it, and a hole cut into the side. As long as it's kept sufficiently wet, you won't need to worry if they're not using the dish.
Hope that helps!