I've been spearfishing since I was a kid. My late father was really talented at it. I have several spearguns and pole spears. My favorite speargun was made by Steve at
www.poseidonsub.com. It's his Voodoo Stick model (I know---sounds corny but it's a handcrafted work of art).
For a beginner, I'd start with a pole spear (aka sling spear) then work up to an inexpensive speargun of moderate size (maybe 100-120 cm with one large or two thinner power bands). If you like the sport, there are quite a few good custom guns discussed on the various spearfishing websites.
One technique is to use cover, whether rock formations, kelp or whatever, to facilitate a stealthy approach on a promising looking spot. If you see something good to eat when you emerge from the cover, get off your shot as quickly as you can aim because fish won't hang around once they spot you.
Another technique is to bait or chum an area by breaking up some urchins/shellfish then coming back a few minutes later to see what is feeding there. Again, concealing your approach is very helpful.
DancesWithKnives