You will need to carry a dive knife with you every time that you go spearfishing. The Scuba Doctor stocks a range of quality spear fishing knives but, like anything, you get what you pay for. A cheaper dive knife will often become blunt and rust quite quickly! It is a good idea to budget $50-$70 for a high quality spearfishing knife that will last you a lifetime.
A dagger is perfect for spearfishing as it has a pointed tip to spike your fish, a smoothly sharpened blade for slicing fish for burly, and a serrated edge for cutting through ropes. It is also important to consider how the dive knife is strapped and how it can be taken out for use.