score:14
The LDS church proscribes wine (or any alcoholic beverage, really), so water is used instead, although it doesn't matter what is used for the symbols of the sacrament, according to Doctrine and Covenants in the LDS canon:
D&C 27:2
2 For, behold, I say unto you, that it mattereth not what ye shall eat or what ye shall drink when ye partake of the sacrament, if it so be that ye do it with an eye single to my gloryβremembering unto the Father my body which was laid down for you, and my blood which was shed for the remission of your sins.
It just so happens that regular bread and water are more available and make for a simpler ceremony, especially considering that it's usually the young men in the church who prepare and administer that ordinance.
There are accounts of Latter-day Saints using alternative emblems for the sacrament. The citation I linked to, about halfway down, mentions French saints making use of potato peelings when that was all they had available.
Note: The Word of Wisdom, which is the revelation forbidding the consumption of alcoholic beverages, was not received until later. It could be that this revelation about the emblems was a precursor, in part, to that more general revelation. (Read D&C 27 in context for more information.)
Upvote:2
During the persecution period of the early Church, the Lord directed Joseph Smith that evil men were seeking to poison the Sacramental wine. Therefore the Saints were to make their own grape juice. Since that became hard to do at they were being expelled from areas, the Church was directed to use water. And since the substance was symbolic anyway, crackers, any style of leavened bread, or in the case of war-torn areas where bread was unavailable, any substitute that would still symbolize the partaker's "doing this in rememberance of Him".