3 links
HowStuffWorks: How SCUBA Works
howstuffworks.com
Photographic tour of everything you need to know about SCUBA and dive equipment, and how your body reacts to the underwater environment.
Scuba Board
scubaboard.com
Articles and message boards about scuba diving, underwater photography and equipment.
Undercurrent
undercurrent.org
Scuba divers' magazine. Online version has a few select articles, in "latest issues" and "back issues index", some of which cover equipment reviews.