An ID book for fish is the best way to learn what kind of fish you're seeing underwater, learn about its habits, and how to distinguish between similar fishes (fun fact - "fish" is a plural for lots of the same species, as in "school of fish" -- while "fishes," though it seems archaic, is in fact grammatically correct when referring to multiple species). Some guide books also offer information about fishing restrictions, and laws set in place for conservation practices. Get your hands on a fish ID book, and impress your buddies by pointing out the difference between the blue-striped grunt and the French grunt -- hint: it isn't the accent.