The water is a necessary ingredient to having a successful roll cast. The water is used to load the rod since a back cast can not be used. With this in mind, practice this cast on a local pond or lake. Even a swimming pool would suffice. To make a roll cast, bring the rod slowly behind you so the line lays at your side in the water.
Most people feel that a roll cast is a different stroke then the overhead cast.
Contrary, the same principles apply to this cast. The resistance of the water will load the rod and help propel the line forward. Most fly-fishing can be accomplished with the basic overhead cast. This must be practiced to understand the loading and unloading of the fly rod. Practicing anywhere there is enough room will help insure proper technique and line handling.