Well truth be told if you're going to have sand, it would be better for damp/ wet sand for it to be held down. After that you'd have to worry about humidity which I hear is bad...?
Take my post with a grain of salt, I'm a leo n00b =P
Damp sand would be very likely to grow mold and/or cause dermatitis in the gecko. They need dry substrate, with a specific hide that is moist for shedding (which can also grow mold if left too long between cleanings).