To many this may seem to be a bit of overkill, but I make my choice based on my diving conditions.
On ocean dives where there is very little, if any, chance of entanglement, I carry a small knife on my left side waist strap.
When I'm diving in my primary dive environment, Lake Lanier north of Atlanta, I carry the aforementioned knife, along with a "standard" dive knife with line cutting notch inside my right calf and a "Z"-knife attached to my right shoulder strap directly beneath my backup light.
There is a seriously significant amount of fishing line and entanglement material in this impoundment and have no desire to spend eternity at the bottom of the lake.