WSU Beach Watchers is a great resource for beginners. It covers the most common seaweeds, sea grasses, shoreline plants, and sea critters in the Salish Sea with good photographs and brief descriptions.
Seaweeds of Alaska has exhaustive coverage of Alaska and includes most of the species found in the Pacific Northwest. The photographs are excellent. Accounts are searchable by both common and scientific name, and include habitat and distribution.
Algaebase is a global seaweed database design for Phycologists (people that study seaweed), especially those concerned with taxonomy. Course level (by country) distribution maps included with some accounts.
Seaweeds of Alaska by Mandy Linderberg and Sandra Lindstrom include all the common seaweeds in the Pacific Northwest. The color photographs show the parts needed for ID.
Marine Life of the Pacific Northwest by Andy Lamb and Bernard Hanby has spectacular photos of all sorts of sea critters and beach life. Expensive but worthwhile to the serious beachcomber and maritime forager.