This will appear as a 1 gallon water skin which, when filled with water or other fluid, will still be light to carry and unencumbering. In fact, the character will be able to carry up 100 gallons of water or liquid in this skin, before it is full, and it never seems heavy or difficult to bring with them on long treks.
The curse comes in one day after the skin has been filled. After 24 hours, the water in the skin turns into bone dry and very fine desert sand. What's more, all similar such vessels within a fifty foot radius (water skins, bottles, etc.) that are carrying water, wine, beer, etc. will also be filled with sand. If the sand is poured out, the water skins or bottles may be refilled with water, but within 24 hours, this effect reoccurs. It will, in fact, continue to happen, so long as the cursed water skin remains within the group.
Because all water vessels become filled with sand at the same time, it may not be obvious where the source of the curse lies at first. The skin has no effect upon springs, fountains, rivers, etc., and it may be discarded at any time.