Bottled spring water or treated tap water can be used. Do not use distilled or plain bottled drinking water. If using tap water, it must be treated with a product which removes chlorines, chloramines, and ammonias. I recommend a product called AmQuel for treating the water. While letting the water sit out open to air for 24 - 48 hours had been accepted as rendering the water safe for use with frogs, few water supples now contain small enough amounts of these chemicals for them to evaporate completely, therefore all tap water should be treated before use.
Water should be changed daily or when soiled to prevent the build up of bacteria in the water which can lead to illness.
If using a bowl for the water area, you will need one which is large enough for the frog to lay in, easy to get out of, and shallow enough to only cover his legs. If using a divided water area, the depth should be the same, and if the area is large - rocks, plants, etc. should be placed about in the area so the frog always has an easy way out of the water. There are several species of frogs which are not good swimmers and they can drown.