It's already a major issue for wineries in California. I know that faculty at UC Davis are now considering water needs and ability to survive drought as an important factor when selecting rootstocks for new vineyard plantings. Here in water-abundant Canada, this has historically not been a major concern. But an article in Wine Business Monthly Online shines a light on similar Canadian concerns in its review of the Okanagan Sustainable Water Strategy and the implications for wineries in the region. The article points out that winery use of water is a tiny percentage of overall usage and that conversion to drip irrigation is making industry use of the resource even more efficient.
The legal implications of water usage are still generally a backburner issue. However, occasionally, as the article points out, the Province can use its powers under environmental laws to cut off water supplies to users as happened for some Okanagan wineries in 2003 during a drought. In addition, water concerns can block development, whether residential or otherwise (including wineries), if the development threatens to change water usage or conservation patterns.