There are a few great ID books that are very helpful when looking up interesting life photographed on dives. The three main books I refer to often are:
Marine Life of the Pacific Northwest: A Photographic Encyclopedia of Invertebrates, Seaweeds and Selected Fishes by Andy Lamb, and Bernard Hanby. Madeira Park: Harbour Publishing, 2005. An updated online version is also available to subscribers at http://knowbc.com/knowbc/Searching-KnowBC
Beneath Pacific Tides: Subtidal Invertebrates of the West Coast by Gregory C. Jensen, Daniel W. Gotshall, Rebecca E. Flores Miller. Bremerton: MolaMarine, 2018
Crabs and Shrimps of the Pacific Coast: A Guide to Shallow-Water Decapods from Southeastern Alaska to the Mexican Border by Gregory C. Jensen. Bremerton: MolaMarine, 2014
Other books that I refer to and have been helpful over the years are listed below.
Paul Humann with Photographers Howard Hall and Neil McDaniel. Coastal Fish Identification: California to Alaska. Jacksonville: New World Publications, Inc., 1996.
Andy Lamb, and Phil Edgell. Coastal Fishes of the Pacific Northwest. Madeira Park: Harbour Publishing, 1986.
Daniel Gotshall. Guide to Marine Invertebrates: Alaska to Baja California. Monterey: Sea Challengers, 1994.
David W. Behrens. Pacific Coast Nudibranchs: A Guide to the Opisthobranchs Alaska to Baja California. Monterey: Sea Challengers, 1991.
David Wrobel, and Claudia Mills. Pacific Coast Pelagic Invertebrates: A Guide to the Common Gelatinous Animals. Monterey: Sea Challengers, 1998.