Gear: Gun, Stringer, Light (for looking under ledges)
How: Jump off boat, start sinking, load gun (never do at surface), shoot fish (aim to head or lateral line mostly), put on stringer through fish's eyes (unless a triggerfish, then go through mouth and out gills, or they'll bite you wherever they can), close stringer, take out spear from fish, put spear in gun, reload and repeat/repeat/repeat til time to go up. Know what sizes are legal for each species, so gotta be able to ID fish and estimate length correctly before shooting. If you need more info, tune into Spearboard's "Spear One" for his seminars, people travel from out of state just to learn from his and other spearfisherman.
When: Whenever I can, take a gun, and if dinner swims by, I get fresh seafood. After all, I pay around $50 for a day of diving, any fish I get helps keep my food costs down and I KNOW where it came from, what it ate, and how it was treated from sea to dinner plate.
Why is it fun: You learn to be a better diver by learning to multi-task underwater, learn to rely less on your buddy and more on your own skills. Another variable - being attached to a fish - is also introduced, which increases the task difficulty/challenge of diving, as I'd gotten bored with bubbles only watching. I've found a great group of friends and am rarely disappointed in the sportsmanship and level of generosity and helpfulness of hunters. It's a great feeling to know you got dinner "all by yourself" and there's a sense of accomplishment in killing (aka stoning) your first fish with a well-placed shot.