Cornwall (and the South West UK): my go-to identification site is Aphoto Marine which has many species occuring in SW England covered (see also Aphotofauna, Aphotoflora and Aphotofungi). Other local rock pooling blogs are Cornwall Wildlife Trust Shoresearch, Cornish Rock Pools and the Marine Enthusiast. The Salty Scavenger is another great British rock pooling blog. See HydroMotionMedia for underwater film and photography in Cornwall. See for Cornish nature and culture the Cornish Birdblog.
Temperate marine aquaria: few sites are available (although forums are a good source of tank diaries, e.g. this thread). Irish rockpool aquarium is a great blog; Jon Olavs Akvarium was the first blog on European coldwater aquariums and is a great source of inspiration. For a Danish tank see Martins’ Makvaerk. The rockpooling UK site has a nice section on rock pool tanks, Keeping British Marine Life is now defunct but is a record of failures and triumphs in keeping a cold water tank similar to this blog (see also Bicycles and the Sea). See here for an awesome Russian anemone tank. Reef Hobbyist Magazine has a very good article by North American guru Stu Wobbe. The Advanced Aquarists has an article on temperate tanks. If you speak Dutch, check the Noordzee en Koud Zeewater Forum. If you want to go really oldschool, check out ‘The Aquarium: an Unveiling of the Wonders of the Deep Sea’ by Philip Henry Gosse, published in 1854! Practical Fishkeeping is a good general aquarium site (and magazine), Reefbuilders has many product reviews and nano-reef has many active forums, focusing on small tanks with some threads on coldwater tanks. See Aquariumdigest for a lot of useful info on LED lighting.
Identification websites: The Marine Life Information Network, British Marine Life Study Society, UK Coastal Wildlife, British Marine Life Pictures, Encyclopedia of Marine Life of Britain and Ireland and Sealord photography contain many species photo galleries. For Molluscs, see The Conchological Society of Britain and Ireland, Marine Bivalve Shells of the British Isles and here for the excellent Nudibranchs of the British Isles book. For sponges, see the Habitas Sponge Guide. A great collection of beautiful macro photographs of British molluscs and barnacles can be found on Morddyn’s flickr account. The Shark Trust’s ‘The Great Eggcase Hunt‘ page contains a key to mermaid’s purses. For general seaweed information see The Seaweed Site.
Facebook: Facebook is an amazing resource when it comes to help ID-ing (photographed) organisms. General groups: Seasearch Identifications (and Seasearch Cornwall), British Marine Life Study Society, Porcupine Marine Natural History Society and Shorewatch. For beachcombing in the UK see Beachcombing (British Coastline) and The Essential Guide to Beachcombing and the Strandline, see also Rockpooling & Shrimping UK. Specialist groups: Seaweeds of the NE Atlantic, Seaweeds and Algae of Britain and Ireland, NE Atlantic Cnidaria, NE Atlantic Bryozoa, NE Atlantic Tunicata, Crustacea of the NE Atlantic & NW Europe, Echinoderms of the NE Atlantic, NE Atlantic Porifera, British Marine Mollusca, NE Atlantic Nudibranchs and Mollusca of the NE Atlantic & NW Europe. Shoresearch Cornwall is a volunteer programme of the Cornwall Wildlife Trust. For Seagrass conservation activities see Project Seagrass and Community Seagrass Initiative. HydroMotionMedia specializes in Cornish underwater video and photo. The Essential Guide to Rockpooling has many rock pool photo’s and In the Company of Seahorses has info on British seahorses; both pages are coupled to upcoming books. For aquarists, the British Marine Aquarists Association group and the Coldwater Marine Aquarium Owners group are highly recommended, see also the smaller page Wet Thumb. Aerial Cornwall has beautiful videos and photos of the Cornish coast.
Identification books: my favoute guide is Collins Complete Guide to British Marine Wildlife: great photographs and a lot of the more common species covered. For more species and information check out the range of Seasearch guides: my Seaweed guide is most used, my Bryozoans and Hydroids guide most needed and my Sea Anemones and Corals is most beautiful. A Student’s Guide to the Seashore does not have very nice illustrations but has good background information. A must-buy is A Field Guide to Marine Fishes of Wales and Adjacent Waters; over 250 species covered, beautiful photos, very informative descriptions (and loads of extra information on topics such as photographing fish and jellyfish-fish associations) at a very good price. Identification Guide tot the Inshore Fish of the British Isles is a bit more complete, but also more expensive and not all the photographs are of good quality. Beneath Cornish Seas is a small (cheap) paperback with beautiful underwater pictures. The Essential Guide to Beachcombing and the Strandline is a must-have.
Underwater Photography: Underwater Photography Guide is an excellent resource. A beginner’s guide to underwater photography is a good read. This is a guide to underwater photography with the Canon G16. British Marine Life Pictures has many useful links too.
Other: Not cold water but a very cool project straddling the boundaries between art, science, outreach and conservation is Coral Morphologic in Miami and green blue sea is a blog documenting marine life around Hong Kong and further ashore.