Hydrocharitaceae is a plant family that includes a number of aquatic plant species [1], broadly called the Tape-grasses, and includes the well known Canadian Waterweed, Frog's Bit, Elodea, Hydrilla, and Egeria.
The family includes both fresh and marine aquatics. They are found throughout the world in a wide variety of habitats, but most are primarily tropical.
The following genera are usually included in the Hydrocharitaceae.
Links:
[1] http://en.wikipedia.org/wiki/Aquatic_plant