Links

Fishbase (great site for species identification)
http://www.fishbase.org/search.php

Animal Diversity Web (for taxonomic information)
http://animaldiversity.ummz.umich.edu/site/index.html

Ceph base (Octopus, Squid, Cuttlefish, Nautilus)
http://www.cephbase.utmb.edu/

Library of 19th Century Science (some of the best guides are the old ones e.g. the Challenger Report) http://www.19thcenturyscience.org/

Fishbase (Amazing resource for all fish information)
http://filaman.ifm-geomar.de/search.php

World Porifera Database (Sponge information)
http://www.vliz.be/vmdcdata/porifera/index.php

Charles Messing's Crinoid Page (A huge resource of information about crinoids)
http://www.nova.edu/ocean/messing/crinoids/index.html

Marine Fauna of Norway (Often useful in identifying many of the deeper water fauna with a wide depth range)
http://www.seawater.no/fauna/index.htm