The World Bank funds development including a number of projects including projects that provide water to developing countries. The World Bank's Statistics Database contains global and country level data on supply and sanitation, irrigation and drainage, hydro power, and resource management.
The World Health Organization (WHO) division of Water Sanitation and Health (WSH) has a Data and Statistics Division that compiles general data on health and sanitation on a global and country level, including water-specific and water-related data. WHO also hosts a page which provides links to information and data on some more water specific issues such as national water legislation and the WHO's Global Water and Sanitation and Assessment Report.
The United Nations provides links to all of the organizations under the UN that address water issues, including UNEP, which conducts a freshwater environmental data report. This is a good starting point for looking at country level data. Of particular note is Aguastat a database hosted by the Food and Agriculture Organization (FAO) of the United Nations. Among the information presented is country level data on institutions, infrastructure, water use, and climate.
OECD Water Supply and Sanitation Programme assists countries in implementing municipal water reform, tariff reform, financial management of utilities and customer protection and involvement in decision-making.
The World Resources Institute (WRI) is an international think tank whose purpose it is to promote protection of the environment and the worlds resources.They host a database that gathers data, statistics and maps on water, other resources, and demographic statistics. The data is country level with some aggregation by development (e.g. LDC) and continent/region.
The World's Water is a biannual publication by Peter Glieck that addresses current issues in the global management of water and gathers a variety of statistics regarding the worlds water supply. The data web page links to a number of tables constructed for the publication. Most of the data is at the global and country level.
Beverage Marketing Corp. is a consulting firm that provides market reports for all types of beverages. The data they provide is both global and national in scope. They provide Water Market Reports pertaining to bottled water (all reports must be purchased).
Global Water Intelligence is another private firm that provides water market analysis on a international and global scale. They have particularly good data on desalination due to their work with the International Desalination Association to create DesalData.com, a website specifically designed to provide data on desalination.