The most extensive and informative online
resource I've found on sea life is the Coral Realm site,
http://www.coralrealm.com. Visitors to this site have
access to numerous articles and pictures of critters from the planet's
oceans, and members are able to search the site's databases on fish,
invertebrates and sharks.
Another site that provides information on
fish identification techniques is http://www.reef.org/webres/gallery.htm.