Placing a pebble or a small rock in the water dish will help prevent food items such as crickets from drowning in the water. Dead carcass left in water will create bacteria that can kill your tarantula. Do not use a sponge or cotton for this purpose they will only aid in harmful bacteria growth. The insides of the enclosure should be lightly misted with a spray bottle filled with water daily. Do not use a spray bottle that contained toxins.
Tarantulas are very sensitive to chemicals.
Misting is very important for arboreal species because they will drink from the dew that collects upon their webs. Make sure you do not directly spray the tarantula, they will flee with panic.