There are several purposes for the knives we have available here.
Dive knives in general: the foremost reason for a knife is safety. Either to free yourself from entanglement or to free up a boat. A short, serrated edge is usually not so good in emergency situations.
Scuba dive knives are usually larger and mounted to BCDs whereas spearfishing dive knives are strapped to your arm, calf or weight belt.
In spearfishing, you use a knife to spike, to kills fish the most humanely way, and to slice fish for burley and to cut through ropes. A spearfishing knife therefore needs to have a pointed tip, a smooth and sharp blade plus a serrated edge.
And finally, you also want to fillet fish. For this, we have filleting knives in stock.
How much to invest into a dive knife:
If you want to have your knife for a long time, choose at least a mid range one. The cheaper the knife, the quicker it will become rusty and blunt. A good quality knife on the other hand will last you for years and might therefore be a smaller expense overall.