Hi BoneDoc,
Most popular spearfishing fins are plastics, like the Cressi Garas, Cressi Professional or a variety of Omer plastic fins (ice, etc.). Plastics are the most durable fin, though they're the least responsive, meaning that when you kick them they're the slowest to return to their original shape.
The second variety is a fiberglass blade (like Nemo Powerfins), which is more responsive that plastics, though more fragile (and more expensive.) After that you're getting into carbon fibers (like C4's), which are much more responsive, though much more delicate and much more expensive.
I'd recommend Cressi or Omer fins. They're in a decent price range and can take a beating. I used my Cressi Gara 2000's for about 2 years before moving into Carbon fibers.
The other thing to consider is the stiffness of the fin. Stiffer fins will give you more power, but will tire your legs faster and can give you leg cramps. Softer fins won't give as much power, but will let you go all day. You'd probably do best with a medium stiffness for spearing.