A punch to the jaw should do it

In the life and death struggle for underwater dominance, any blow is a good blow. The difference between life and death can be the determination to win. Both divers here are fighting for their lives. One to save the world. The other for the thrill of a underwater kill and the well paid bounty paid by Spectre.  The butt of the dive knife smashed into the body of the enemy can be just as effective as a thrust with the knife blade. Once the enemy has been dominated and in a position of weakness. the Killer blow with the sharp deadly blade of the dive knife can come into play. A well trained trust with the steel razor sharp blade, with the slice of rubber of the enemy's air hose, will take the victory for one diver and leave the other drifting to the ocean floor in a trail of blood and bubbles. Defeated and lifeless.