general	Australia
photography	Egypt
nudibranchs	Japan
	New Zealand
	The Philippines
	The Solomon Islands
	United States

General

FishBase	When I visited in March of 2004, this extraordinary fish database claimed to be getting around 9 million hits a month, and to have 35900 photos of 28400 species of fish.   It's an excellent way of identifying fish, either by following the "identification" link at the top of the front page, or by typing a family name like "butterflyfish" into the "common name" field on the front page.  The site is definitely slanted towards academic research, but it has some wonderful photos in the Best Photos of the Month section.   It allows you to search in several different languages, including Russian, Greek and Arabic.   Last visited in April of 2006.
FINS	Billing itself as the "Fish Information Service", this website has a database with hundreds of good photos of fish and other marine life, listed by type of fish, by common name and by Latin name.   The marine fish by type sections have very good photo galleries of all the types of fish belong to different families, like "angelfish", "eels", "sharks" and so on, making identification of unknown fish a breeze.
Reef Images	This website has great photo galleries which can help you identify various types of creature including sea cucumbers.
Scuba Equipment USA	This scuba equipment retailer won't rate a mention except for their excellent marine species gallery, with separate sections for fish, crustaceans, nudibranchs (197 images at 30 images per page), anemones, corals, mammals and other categories of life from around the world.   The species accounts have the best write-up of any website I've seen anywhere.   Last visited in April of 2006.
Marine Life Images	This stock photography website contains galleries of photos from the Pacific Ocean, the Caribbean and the Galapagos Islands (funny, I thought they were in the Pacific!).   The website isn't as slick as some others, but the galleries can certainly be useful if you need to identify something you've seen.
Yahoo underwater life website links	A categorized list of about 650 websites devoted to all facets of underwater life.

Photography

underwaterphotography.com	The name is a mouthful, but this is a useful website, with forums, a for sale section, and articles covering topics like underwater digital photography and the rise and eventual demise of the Nikonos underwater camera system, which for many years was the only way to do underwater photography.
The World of the Underwater Photography	The English language version of this French language website has lots of articles which aren't well written, but contain a lot of sensible information.
The World of Steve Norvich	This person's website is very well done, with trip reports from about 15 Pacific dive sites and 6 Caribbean sites.   He also has several pages devoted to underwater photography techniques.

Nudibranchs

Poppe images	See the description of the Poppe Images website.
Scuba Equipment USA	See the description of the Scuba Equipment USA website.
Opistobranchs from ... Elsewhere	Opistobranchs are sea slugs or nudibranchs.   I didn't want to immediately present you with the whole name of this website because I thought it might put you off - it's called "Opistobranchs from the Mediterranean and Elsewhere", which is a silly name, since "the Mediterranean and Elsewhere" is everywhere, isn't it?   Anyway, as you can tell it's obviously spread its wings, because it's now got nudibranchs from all over the world.   It might also be offputting to learn that this website is based in Germany, but in fact it has dual German and English sections.   The photo galleries are really well put together, and exceptionally useful when you're looking for a particular sea slug, but there's no description of the species in the galleries.
Marli Wakeling	Marli Wakeling's website has galleries from her foreign trips, featuring life both above water and below the surface.   The site includes a nudibranch photo gallery with a total of 105 thumbnail photos displayed at 40 thumbnails per page, together with the scientific name of each species.   The photos are gorgeous in their own right and it's useful also if you want to identify nudibranchs that you've seen.   Last visited in May of 2006.
Dive-Oz nudibranch galleries	See the description of the Dive-Oz website.
Sea Slug Forum	This website sponsored by the Australian Museum Online is oriented towards the scientific community rather than lay people and, as with many such biology websites, it provides a large linked list of nudibranchs rather than a gallery, which makes using this website for identification very tiresome.   The main value for regular people is likely to be the forums, where you can post photos of nudibranchs to enquire about what species they are, and the long and well-annotated list of other nudibranch websites.

Australia

Dive-Oz

"Oz" is a slang term for "Australia", and Dive-Oz claims to be Australia's biggest scuba diving website.   It's a bit patchy but the discussion forums seem fairly active and specialized parts like the nudibranch galleries are fairly well done, with 12 pages each containing 12 thumbnail photos - but the enlarged photos are only 300 pixels wide.   Last visited in April of 2006.

Egypt

Geographia:  Diving Sinai

This section of the Geographia travel website contains information about dive sites around the Sinai Peninsula.   There isn't a lot of information, but it's worthwhile as an introduction.   The photo gallery of Red Sea fish has too few photos to make it a useful resource, and the photos are far too small to make it worth much time browsing.

Japan

Bob Bolland's Okinawa Sea Slug website

Yes, it's what you've been waiting for all your life, but thought you'd never see - a website dedicated to the nudibranchs of Okinawa!   It also has links to other more general nudibranch websites.

New Zealand

WhaleWatch

WhaleWatch is a marine life tourism company very professionally operated by a local group of New Zealand's indigenous people, the Maori.   Situated at Kaikoura, on the North East Coast of the South Island, they run regular boat trips off the coast to watch some of the sperm whales which are year-round residents of this area.  Using microphones to locate the whales as they dive to great depths for squid, it's almost guaranteed that visitors will be able to see the whales.   Although professionally done and with good graphics, the website is fairly slow and geared towards a very small screen size.   Last visited in January of 2008.

The Philippines

Poppe images

This very professionally presented website is set up as a stock photography business, but has a great deal to offer people who don't want to buy anything.   There are a huge number of photos of underwater wildlife, laid out in very easy to navigate photo galleries - an excellent resource for identifying critters you've seen, such as fish and nudibranchs.   There's also brief information about various dive sites.   Last visited in December of 2005.

Solomon Islands

Taucher.Net	How's your German?   After seven wasted years of study, mine's not too bad, so I was able to glean quite a bit from this German language website devoted to wreck diving, which has a section on the Solomon Islands.
United States LST Association	The United States LST Association is dedicated to the preservation of artifacts related to the LST or "landing ship, tank".   They even have an operating example of an American World War Two LST.

United States

Hawaii Coral Reef Network

This excellent website has photographs and text of Hawaiian marine life, grouped by taxonomic family.   One little quibble is that it's not completely obvious how to get to the photographs from the front page, since the large black text at the bottom - "seaweeds", "invertebrates" and "fish" aren't underlined, and therefore don't look like links.