Water Technology An industry website, with information on current and planned projects and who's doing them.
Centre for Alternative Technology
Wave Energy An overview, with links to wave energy companies.
Tidal Energy An overview, with links to tidal energy companies.
Hydro-Electric Power An overview.
Foundation for Water and Energy Education US hydropower site, featuring a good explanation of how it works.
Environment Agency advice on water-efficient technology for the home, garden and business, with supplier listings.