For those who like to ID everything, (and settle arguments/save marriages), but don't carry heavy books on dive trips, Apple is offering Neville Coleman's Marine Life eGuide
for download to iPhone / iPod touch. C'mon, you know you take it with you. It's a pretty comprehensive guide for Indo-Pacific species, with fantastically helpful photos. Not limited to just bony fish or nudibranchs, it covers flatworms & cephalopods and cone shells and even plants. You do have to put up with some curious Aussie common names
like Butterfly Cod (Lionfish) and 2 totally different species called Coral Rockcod, but that's part of the fun. Neville is one cool dude.