Many resources exist to aid in the identification of aquatic plants.  The Aquatic Nuisance Control program has compiled a list of resources for your information.

Aquatic Plant Identification and Herbicide Use Guide, Technical Report A-88-9
Department of the Army, Waterways Experiment Station, Corps of Engineers, P.O. Box 631, Vicksburg, MS 39180-0631
Limited Supply

How to Identify and Control Water Weeds and Algae
Cygnet Enterprises, 1860 Bagwell Street, Flint, MI 48503
Contact:  800-359-7531 or cygnetent@aol.com
$9.95 + $3.00 S/H

Through the Looking Glass...A Field Guide to Aquatic Plants
Wisconsin Lakes Partnership, University of Wisconsin-Stevens Point, College of Natural Resources
1900 Franklin Street, Stevens Point, WI 54481
Contact:  715-346-2166 or http://www.uwsp.edu/cnr/uwexlakes/publications/fieldguide

Aquatic and Wetland Plants of Northeastern North America, Volumes 1 and 2
By Garrett E. Crow and C. Barre Hellquist
The University of Wisconsin Press

Michigan Flora, Volumes 1, 2, and 3
By Edward Voss
University of Michigan

A Guide to Aquatic Plants: Identification and Management
Division of Ecological Services, Minnesota Department of Natural Resources, 500 Lafayette Road, St. Paul, MN 55155-4025
Contact: 651-259-5100 or http://www.dnr.state.mn.us

Aquatic Plants: Their Identification and Management
Illinois Department of Natural Resources, One Resources Way, Springfield, IL 62702-1271
Contact: 217-782-6302 or http://www.dnr.state.il.us

Grasses, Sedges, and Rushes Identification Deck
IFAS Extension Bookstore
University of Florida
P.O. Box 110011
Gainesville, FL 32611-0011
Contact: 800-223-1764 or http://IFASbooks.ufl.edu