Summarize:

Geckos should be supplied with a dish of water that they can drink. Make sure that the water is replaced and the dish cleaned on a regular basis, ideally every day. Provide tap water treated with Reptisafe, tap water left out for 24 hours, or spring water. Never distilled as this lacks minerals. If your tap water doesn't contain chlorine (which is the case in countries such as the United Kingdom and Germany)it can be given to your gecko as it is. This water should not be very deep, so that there is little chance of the gecko drowning in it.
Give your leopard gecko water.