NASA
Climate Kids: 10 Interesting Things About Water
Discover 10 very interesting things about water, including sources of life, ocean ecosystems, freshwater and ice, salt and salt water, sources of water, the body's water makeup, plant's consumption of water and the 3 states of water.
Microscopy UK
Microscopy Uk: A Virtual Pond Dip
In this site, you will be introduced to some of the more common organisms found in a pond. Contains a thorough guide to small and microscopic pond life with links to other descriptive sources.
Other
Beloit College: K 12 Geology
This is a good source of basic information about density currents in water. It includes simple activities you can do to demonstrate these currents.